how to add to install image?

How to add extra sets to the installation?

I see SET_MD_1_NAME to SET_MD_4_NAME to add to the sysinst sets menu but 
don't see documented. Is that what I need for sysinst?

Also how do I tell the "release" and iso-image (and other) targets about 
my additional set(s)? I assume I can just manually put them in place, 
but maybe there is some variable or setting to define also?

SETS overrides the default sets (so be sure to include all)

What copies them to SETS_DIR? (I do manually?)

IMGFILE_EXTRA  to copy a file  (maybe I could use that)
IMGDIR_EXTRA  to copy a directory

I probably don't need to use src/distrib/sets/maketars
or src/distrib/sets/sets.subr to generate my own sets.

Maybe I don't need to use "sets", but I do want to bundle installed 
packages (including /var/db/pkg) or ready-to-install packages and have 
them installed during installation.