Indirect belly heat is where the heat cable is directly behind the boxes instead of underneath them, so it provides a bit of a warmer ambient temp if necessary. AP generally recommends it for their smallest hatchling racks, since baby boxes are usually too small to allow for a big thermal gradient so going with direct belly heat may provide too much warmth.

I have several AP racks & would go with direct belly heat unless you're getting a hatchling rack.

Just my $.02

K~