It's just a way of grouping similar functions together.

How do you know which modify key you need to hit...cntrl, option, shift etc

You need to search the Digidesign website, or look in your manual for the key commands.

When you go into the menus, each function should show it's equivalent key comand. Other than that, I believe there is a printable map that you can download from digi's site. I'm just learning protools so I've been using the support pages a lot lately. :oops: