"Unit Features" are additions to basic units in C-evo. The manual divides them into three groups:
Some unit features (such as Jet Engines) are automatically incorporated in any unit design of the appropriate class(es); you don't have a choice of saying "No", even if there are matching costs (such as production cost or weight) built in.
Others have to be specifically requested, with the matching costs (such as production cost or weight) built in.
The manual page that lists them all is headed "Unit Features" and is item 7 on the main "Manual" menu (the first thing you see when hitting "F1" or the book icon at top left of another manual page. You cannot find it by searching for "Unit Features" or "unit feature".