POSIX specifies the API available to programs, and also shell and commands and stuff available to users. It does not specify which functions should be available from the standard C library, and which should be available from the kernel: from the standpoint of POSIX, it’s all the same. POSIX doesn’t care how the API is implemented, just that it is implemented correctly.

-X is already included in -a, so no need to specify expliticly. Doesn’t hurt either.

Nope, I was wrong, -X is not included in -a. Sorry!

-x (alias --one-file-system) means “don’t cross filesystem boundaries”; is that what you meant? Or did you mean -X | --xattrs?

Edited because I wrote some things before that were incorrect.