Slitherine

I was using the L hot key last nite and was wondering what the firepower rating means:

The situation was a mobile Artillery unit that attacked an enemy adjacent to it...
The L key explained the artillery had a firepower rating of 90%
havnt noticed any other units having this.

Is this exclusive to artillery and (complete guess) maybe only used when they are adjacent to an enemy unit (as opposed to long range strikes?) Simulates less effective direct fire vs ballistic?

From what I can tell this comes from the "Rate of Fire" column in the unit-tables. It caps how much firepower a unit can have regardless of size. Or so I've been told...