Many different specifications (RFCs) over the long history of FTP describe its official implementation. FTP is about 40 years old, but there are about 10 "newer" specifications written over the last 25 years that implementors need to take into account, only some of which are linked to each other. You might not find them, even if you know what you're looking for! :)
In 2010, an official registry of "FTP Commands and Extensions" was created. While general users of FTP won't have a use for this, it can be very handy for implementors to have them all gathered together in one spot as a reference. Any new commands are required to be listed there.
FTP commands are listed by categories of mandatory to implement, optional, or deprecated, with their specifications for more details on them.