Editable source, not just a flat file
The working files come with the finished document, in a format you already have.
Nothing is locked, and there is no tool you have to licence to open your own text.
Numbering that survives revision
Sections, figures and tables are numbered so that inserting one in the middle does not
renumber every cross-reference behind it. Revisions are dated and listed on one page.
Illustrations as separate files
Line drawings and diagrams are drawn and handed over on their own, so a part can be
redrawn without rebuilding the page it sits on.
A terminology list
One name per thing, agreed with you and written down, so the panel is not a console on
page 12 and a display on page 40.
Ready for translation
Text is written to be translated: short sentences, no idiom, no text baked into the
pictures. The translation itself is somebody else's job, and that is said plainly.
A short style sheet
The rules the document was written to — headings, warnings, units, how steps are
phrased — so the next person who edits it does not pull it out of shape.