Dominions 5 Wiki

A unit with a Supply Bonus gives into the Supply pool of wherever they are at the moment. They either cause crops to grow more profusely, or they yield something that can be consumed – milk, perhaps, or maybe even meat.

This offsets their Supply Usage, so what they actually give in is less than pictured; unless they need not eat, of course.

In Dominions 5, Gluttony and Supply Bonus are essentially the same stat, except that one is positive while the other is negative.