I actually don't think this is any clearer.

It takes one clear but non-standard concept ("importance") and turns it into a less clear but more standard concept ("impression").

I think the average user won't know what an impression is and definitely won't understand what an "impression modulus" is.


We could rename it, but "modulus" is the confusing part really.

Maybe "Display likelihood"? And then a description that included "A likelihood factor for this banner being displayed, relative to the likelihood factors of other banners."

Okay you do make a point about "modulus" being a confusing term as well. Display likelihood sounds reasonable.