I really wanted to know because it would really help with my projects.

If I associate the term 'garbage matte' correctly you might want to have a look at the rotoscoping filter.
Otherwise please explain the term.

Don't you just hate it when a user appears, asks a question and never returns?

Yeah, me too.

A garbage matte is just a quick way of masking out a bit of the screen that we don't want to see. Typically, it might be the raggy edges of your green screen or something like that but there's no detail there that you need to roto. That's why they're called "garbage" mattes - because they're something you're going to throw away.

NOW - there's another (drum-roll please) reason why you might want use a garbage matte and that's the one that brought me here. Such mattes are temporarily used to mask out little bits of the screen for the purpose of colour correction. Skintones are vital to get right for a professional finish unless you're going for some weird look (see "Welcome to the Punch" for an egregious example).

We measure skin on the vectorscope and there's a line at (about 340 degrees) there where all skin lives - no matter how light or dark (and it has to do with the way light reacts with flesh or some such) but to SEE if your skintones are on the line, you need to isolate them so there's no extraneous noise on the image filling the scopes with all the other crud in the scene. In effect, unless you're doing an adult movie with a close-up of the thespians (cough) there's gonna be a LOT of it.

Hence some way of isolating these on the scope is really important and I think that's what the OP was after here.

Quick mattes are pretty important and although I guess we could use a vignette effect, that's a bit clumsy. Perhaps this is something someone else can clear up?

The "Rectangular Alpha Mask" effect works fairly well. Use the Effect Properties window to create the mask (rather than drawing the mask in the preview monitor).
Its an alpha mask so if you apply it to a video clip on a track above another video track that has a clip in it, you will see the lower clip show through as you create the mask. To get around this, I simply "mute" the lower the video track so it doesn't show through the mask. This is done by clicking on the little movie frame icon in the track title box which is to the left of the tracks.
It seems to work well enough.

"Alpha shapes" gives you a bit more choices of the mask shape

markoc wrote:"Alpha shapes" gives you a bit more choices of the mask shape

Ah! Great, I'll check that out. Thanks for the tip!