15 pound felt is the "default" for most applications.
30 pound is used for lower slopes (under 4:12 where shingles are permissible) with a "double lap", i.e. lay one course, then where you'd normally lay a 'normal' course about 3" down overlapping the felt below, a double lap is where the top layer starts half way up the bottom layer (I hope that made sense).
OK, so I started this response about 2p & am finishing this detail here @ 10:35 cst.
This detail should show the alignment of a low slope, double lapped 30 # application.
For slopes 8:12 & steeper, 30 # is the typical felt used because it's thicker & less prone to tearing when walked on or worked on / around / with.
Now let's see who has responded since I started...