APBRmetrics

Is defensive RAPM an estimate of the net defensive +/- contributed by a player in comparison to an average player? Or is it in comparison to a replacement level player? Or is it simply in comparison to the replacements available to the team of the particular player of interest?

To the best of my knowledge it is above average.

I.e. Dirk being +4.8 on offense and +3.2 on defense turns an average NBA possession from 104.6 (this year) on offense and defense into:

Offense=(104.6+4.8) = 109.4
Defense=(104.6-3.2) = 101.4

When you do APM or RAPM the mean is close to zero, which we assume is the "average" rating.

There are 198 players this year with ORtg > 104.6
Then there are 280 who were < 104.6
The median ORtg is about 102: half are higher, and half are lower.

Median is 8th or 9th best on a typical team; average is 6th or 7th.
RAPM (or any APM) should not be relative to a team, but to league average, which is zero.