In the manifolds' descriptions, we have used the following abbreviations:
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. Barb fittings can be either smooth or ribbed, and they generally work best with flexible pipe from the same manufacturer as the fitting.
Here are the main pipe sizes in millimetres :