A reminder of the different types of plumbing fitting:
F: a female or "socket" opening, into which you can glue either a pipe or a male fitting.
M: a male, or "spigot" fitting, with the same external diameter as the corresponding pipe, which will slide inside a female fitting of the same type.
M+: a "barb" fitting which is designed to slide inside a pipe. This type of fitting, which can be smooth or ribbed, is very useful if you cannot glue a pipe into a female fitting, du to lack of space, for example, and works best with small pipes (especially 1 inch and less) from the same manufacturer as the fitting.