ROT_PA is the Mechanical position. In your ACP logs you will see

Imager sky position angle is xxx deg.
Rotator mech. position angle is yyy deg.

ROT_PA is the latter number, yyy, the derotated mech angle. Use this for flat fielding. If they are the same, then your (derotated) rotator happens to index at 0 against the equatorial coordinates system ([fixed-offset] = 0). ACP will handle rotators that have any [fixed offset]. And the [fixed offset] never changes, it is at the derotated side of the rotator.

If you need access to the "raw" angle (for example to handle vignetting in the primary optics) you'll need to get that from PlaneWave. Welcome to the world of imaging on an alt-az mounted scope. You also have no access to the max-rotation-speed area around the zenith, and depending on the model, long "slew" times while the derotator catches up to the fast slews of the scope, particularly near the zenith. ACP sees slew completion when all three axes arrive at the requested equatorial coordinates. Most astronomers eventually re-mount the scope on an equatorial "wedge" or equivalent.