U3D (3D)

U3D (Universal 3D) is the 3D format of the standard ECMA-363, made for 3D in documents: it was the first format 3D PDFs held, and Adobe Acrobat and Reader still show it. A U3D file holds a scene: its parts, each where it stands and in its own colours and textures, and animations that move them. ParametrIQ CAD writes a 3D page as U3D, folding animation and all, puts it in a 3D PDF, and reads U3D models to show with a folded drawing.

Export

3D › Export › Export 3D Model…, choosing U3D (.u3d), writes the 3D page as it is shown: folded as far as it is now, with its model.

A part for each panel. Each panel is a part of its own (a node, in U3D's terms), named as the 3D page names it and placed where it is shown, so a program that shows the file can move it on its own.
The fold. The file holds the fold as an animation, from flat to folded in 1.5 seconds, slow at first, faster, then slow, as the 3D page folds: each panel's node turns and moves about its crease. A fold's bend, and a part that bends, change shape as they fold, which a node can't: each shape the bend takes is a part of its own, shown while the fold is at that shape. Programs that don't play animations show the design as it stands.
Its faces. Each part holds its outside, inside and cut edges, in the appearance's colours, opacity and gloss.
Its artwork. The artwork printed on the outside and inside is a texture (a PNG, its sides scaled to a power of two, at most 2048 pixels), printed over the board's colour and mapped as the 3D page maps it.
The model. A 3D model shown on the page is written too, with its colours and textures. It doesn't move.
Lights and camera. A soft ambient light and two lights from above, and a camera where the 3D page's view is.
Units. The file is in millimetres, Z up, whatever the drawing's units, and says so.

Foil, varnish, emboss, the board's structure and its surface patterns, which the 3D page draws with its own shading, aren't written; nor are the floor and its shadows.

In a 3D PDF

File › Export › Export 3D PDF… puts the 3D page in a PDF page that Adobe Acrobat and Reader show in 3D, folding when the page opens. With U3D as its 3D format, it holds the U3D file above, its fold played by the PDF's own animation settings. See 3D PDF.

Import

3D › Models and Products › Import Model… reads a .u3d file and shows it with the folded drawing, as it does a glTF, OBJ or STL model (see 3D pages). What is read:

each model where its nodes put it (the first place, for a node with several), its meshes complete: most U3D files send a mesh as a series of refinements, each adding a corner to the one before, and all of them are read;
each part's material: its shader's colour and opacity, and its first texture (a PNG or JPEG in the file);
colours given at each corner are shown as one colour for the part (their average);
the file's units, when it says them; a file that doesn't is taken as millimetres, unless it is under 10 units across (then metres). U3D has no "up": Z is taken as up, as Acrobat takes it.

Left out, and said in the message after the import: point sets, line sets, text (glyphs), bones and skin animation, subdivision, other files a file refers to, textures in other files, and blocks of extensions. A mesh that can't be read to the end keeps what was read. The model doesn't move: its animations aren't played.

A PDF holding a U3D model isn't read as a model: export the U3D from it with Acrobat, or from the program that made it.

About the format

The standard, ECMA-363 (4th edition, 2007), is free to read. A U3D file is a series of blocks: a file header, then declarations (the nodes, each with its modifiers, the meshes, shaders, materials, textures and motions), then continuations (the meshes' data and the textures' images). Many values are compressed with the standard's arithmetic coder; ParametrIQ CAD writes each mesh whole (its base mesh, its faces' corners compressed) and reads both kinds of mesh. Its files say they are version 0, as programs built on the standard's reference implementation (Acrobat among them) require.