Can someone tell me what the difference is between the Square D breaker FA36100 and the Square D breaker FH36100?
These both apparently fit the I-Line panelboards, are rated for 600 volts and have the same physical dimensions.

From what I can cipher from the catalog, for 3 phase:
FH designates a 65K interrupting rating, @240V, and 25K, @277/480V and 480V;
FA is good for 25K, @240V, and 18K, @ 277/480V or 480V.
The "H" stands for "high interrupting rating".