Explainer
When the Source of Truth Is a Model
The Shift
From Drawings to Definitions
The technical data package (the contractual definition of a product, governed by MIL-STD-31000, now at Revision C (2025)) comes in two types: the traditional 2D drawing package, and a 3D type where the definition lives in digitally connected model files. The 3D approach, Model-Based Definition, makes the annotated model itself authoritative; drawings, where they exist at all, are derived views. DoD's digital-engineering policy direction reinforces the same principle: models as the source of truth, kept accessible and usable for decades. The much-invoked digital thread is this idea extended through the lifecycle: design data flowing to manufacturing, sustainment, and yes, documentation, without being re-keyed at each border.
Technical publications have always lived at the end of that chain: someone read the drawings and wrote the manual. The question the model-based shift poses is whether that hand-off stays human transcription, or becomes a data flow.
The Authoritative Record
When the Model Becomes the Document of Record
People reach for various shorthand when they describe this, and the one you hear most often is some version of the model as the legal document. It is worth being precise about what is and is not standardized there, because the phrase gets used loosely. There is no single specification with that title. What exists is a body of standards that make an annotated model a complete product definition in its own right, so that it carries the authority a drawing used to carry.
The central one is ASME Y14.41, currently at its 2019 revision and first issued back in 2003, in anticipation of 2D drawings being phased out. It sets the requirements for preparing and revising digital product definition data: how dimensions, tolerances, notes and material specifications are embedded in the model itself as product manufacturing information rather than living on a separate sheet. Its international counterpart is ISO 16792, and both define a data set as the thing that constitutes a complete product definition: the annotated model plus its related documentation. ASME Y14.47 adds model organization practices on top. The Department of Defense has adopted Y14.41, which is how the requirement reaches defense programs.
The word doing the work in all of this is complete. A model becomes the record not because it is three-dimensional but because it is annotated to the point that nothing essential lives anywhere else. Until that threshold is met, a program is running a model and a drawing set and quietly depending on the drawing, which is the most common real-world state and the least discussed.
For a publications team the consequence is specific rather than philosophical. If the model is the authoritative definition, then the figures in your manual and the parts data behind your component documentation are derivatives of a controlled artifact, and they inherit its revision state. A change to the model is a change to your source, whether or not anyone tells you. Programs that treat illustration as an artistic task rather than a derived one discover this at the worst possible moment, which is when a maintainer notices the figure disagrees with the hardware.
The Last Mile
Publications Are the Thread's Delivery Layer
The breathless version of digital-thread rhetoric implies the model replaces the manual. It won't. A maintainer needs a validated procedure with warnings in the right order, filtered to their configuration, on a device that survives a flight line. What changes is the manual's supply chain: illustrations generated from model geometry rather than redrawn, parts data flowing from the model's structure into IPD modules, procedures tracing to the design elements they touch. The S-Series family already runs this philosophy, analysis data upstream, publications downstream, one configuration view, and model-based TDPs extend it to the design definition itself.
The readiness test for a tech pubs organization is unglamorous: connection requires structure on both ends. Publication source that is structured and addressable, identifiers aligned so content can reference design data, identifiers are architecture, again, and illustration pipelines treated as the first practical bridge, because model-derived graphics are the mature piece today. Organizations still authoring from PDFs of drawings aren't one step from the thread; they're two, and the first step is the same structured foundation every other future in this series requires. The thread rewards the prepared.
FAQ
Questions We Hear
What does MBD stand for?
MBD stands for Model-Based Definition. The full form is the whole idea in three words: the definition of the product lives in the model itself, annotated with the information a drawing used to carry, rather than in drawings derived from it.
What does it mean for the model to be the legal or authoritative document?
It means the annotated 3D model, rather than a drawing derived from it, is the controlled definition of the product. There is no single standard called 'the model as a legal document'; the phrase is industry shorthand. What underpins it is ASME Y14.41, currently at its 2019 revision, which sets out how product manufacturing information (dimensions, tolerances, notes, material specifications) is embedded in the model so that the model plus its related documentation forms a complete product definition. ISO 16792 is the international counterpart, ASME Y14.47 covers model organization, and the Department of Defense has adopted Y14.41. The practical test is completeness: the model is the record only when nothing essential lives anywhere else.
Does model-based definition remove drawings entirely?
That was the intent when Y14.41 was first issued in 2003, and in practice most programs are somewhere in between. Plenty run an annotated model alongside a drawing set and rely on the drawing when there is any doubt, which is a reasonable transitional state but means the model is not yet the authoritative record whatever the policy says. For downstream consumers like publications, the question worth asking is not whether the program has adopted MBD but which artifact the engineering organization actually treats as correct when the two disagree.
What is a model-based technical data package?
MIL-STD-31000 (the standard defining technical data package requirements, now at Revision C, issued February 2025) has recognized two TDP types since its first edition in 2009: the traditional 2D drawing-based package, and a 3D type where the product definition lives in digitally connected model files. In the 3D approach, called Model-Based Definition, the annotated model is the authoritative product definition rather than drawings derived from it. DoD's broader digital-engineering policy direction pushes the same way: the model as the source of truth, kept accessible and usable for decades.
What does that mean for technical manuals?
It relocates their upstream source. Manuals have always been derived from product definition; someone read drawings and wrote procedures. When the definition is a model, derivation can become partly a data flow: illustrations generated from model geometry, parts information drawn from the model's structure, references that trace back to model elements. The manual stops being a hand-made interpretation of the design and starts being a downstream product of it, which is exactly how the S-Series family already treats the relationship between analysis data and publications.
Does the model replace the publication?
No, and this is the trap in the more breathless digital-thread rhetoric. A maintainer on a flight line needs a validated procedure with warnings in the right places, filtered to their configuration, on a rugged device, not a CAD session. The publication remains the product the human consumes; what changes is its supply chain. The realistic architecture keeps structured publications (S1000D data modules and their kin) as the delivery layer, increasingly fed and updated from model data rather than transcribed by hand.
What should tech pubs organizations do about this now?
Position for connection rather than transcription. Concretely: keep publication source structured and addressable (a prerequisite for any automated feed), align identifiers so publication content can reference design data (the same 'identifiers are architecture' lesson as the DMC), and treat illustration pipelines as the first practical bridge: model-derived graphics are the most mature piece of the story. Organizations still authoring from PDFs of drawings have a two-step journey; the first step is the same structured-content foundation everything else in this series requires.
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