I have a BT-99 that came with a Graco adjustable butt plate. I have not been able to find anything that explains how to use this adjustability to correct POI errors.

It also has an adjustable comb and I understand how to use that feature. Moving the comb up moves the pattern up. Moving the comb right moves the pattern right, etc.

But I have not been able to find anything that explains the effect of moving the butt plate one way or another. Is there any reference that explains how to use an adjustable butt plate?

The butt plate is for comfort and promoting a consistent mount. It has nothing to do with POI. You adjust the POI with the comb and/or rib (if that's adjustable).

Inconsistent mounting moves the POI around. An adjustable butt-pad is about comfort and consistent stock mounting.

Toe angle should match your shoulder pocket. On your stock shoulder, raise that arm to simulate a shooting position. with your other hand feel your natural stock pocket and angle configuration. Match the butt pad angles to your natural shoulder pocket.

Pad position higher or lower should match your neck length. The best way to judge that is mount the gun from off shoulder to shooting position. Assume your mounting position and bring the gun to your dominate eye without contorting head angle, neck position or shoulder raise. Set the pad adjuster vertically so to allow a one move mount without body contortions.

Length of pull (LOP) should allow raising to shooting position without catching on clothing or arm pit. Not too short hitting your nose with you thumb every shot. A little short (1/8th inch) is OK allowing more free gun movement to the target. If you find that your cheek and back sight (dominate eye) moves from mount to the target there is something wrong with the LOP and gun fit.

Key word .... comfort. Key result .... less felt recoil and or cheek pain.
Learn basic gun fitting.

Maltz

Another way to look at it which is consistent with Maltz’s advice is to mount the comb to the cheek pocket with the head held as preferred by the shooter and then slide it back to the shoulder pocket. The butt pad will allow adjustment for the height of the butt. Rollin Oswald is not opposed to the butt being as much as an inch above the clavicle although somewhat lower than that is more normal and should be possible with the adjustable pad.