Pavg controls or stores certain properties in the following elements:  GLAS, QRTZ, TUNG, STOR, virus (solid, liquid, or gas), PIPE, and PPIP.  Here is a schematic on what I know about pavg:

Virus:

Pavg0 controls the speed at which the virus is cured.

Pavg1 controls the speed at which the virus dies.

STOR/PIPE/PPIP:

Pavg0 stores the tmp of the captured particle.

Pavg1 stores the ctype of the captured particle.

Pavg also controls the breakability mechanics of GLAS, QRTZ, and TUNG.  I don't know how that works though.

Like flags, pavg isn't displayed in the hud, even in debug mode.  Unlike flags, pavg can be published in a save.