In technical and regulatory contexts, "Part 21" refers to a critical set of aviation standards. If your file is a digital copy of these regulations, here is an informative overview:
: Multi-part RAR files require all segments to be present in the same folder. If "part21" is the final part, you will also need the preceding 20 files to extract the content.
: Formal document stating the specific aircraft is safe to fly.
: Approval of the aircraft's design and components.
: Used when someone other than the original manufacturer modifies an aircraft's design. Technical Review of the .rar File