High and Low side a/c

67Dart273 is spot on.

This is a little off base, but wanted to add, if you get a jug of 134a from a commercial supply house, it comes with the standard 1/4in. flare fitting, vs the 5/16? fitting used in automotive.
So with a commercial jug, you can use a standard r12 guage set for charging..... Not to mention, it is much cheaper....