The part17 specifically means this is the of that archive. Without all the other parts (part01, part02, …, partXX), this file alone is completely unusable.
Organized files are easier to back up and recover. Knowing exactly which files you need to save and being able to locate them easily ensures that you can quickly restore your data in case of a loss.
: This is the unique ID number assigned to a specific video or "title" within the FC2 database.
(part01 through partXX) to successfully extract the original content. Missing even one part will result in an error.
This guide provides a general approach to handling multi-part RAR archives. If you encounter specific issues, consider checking forums or the documentation for the extraction software you're using for more tailored advice.