/etc/portage/sets

For each file in, a package set is created, defining a one-to-many relationship between a set name and an arbitrary group of package atoms. User-defined set names correspond to the name of the file. Each file should contain a list of package atoms and nested package sets, one per line. You can see the full list of sets that exist at any time with

Example
Say you were running live ebuilds for your GPU drivers, and wanted an easy shorthand for rebuilding them on demand. Define this set:

Now, all you need to do to trigger a rebuild of all of them at once is