Slitherine

When you disband units you regain manpower points and production points. These two numbers show as plus notations on the pop-up menu. But the top number is a minus notation, and it shows as an icon that is a chest and head of a soldier.

What is it?

If I've understood what you're saying, I'm pretty sure it's the number of health points you're disbanding.

What are "health" points?

just its remaining amount of strength points. Not really useful info, the tooltip was based on the repair one which displays strength points gained, then PP and MP cost.