PPXX, where XX is numerical value available from 00 through 32 (there may be couple more higher values further from there, skipping many unused values along the way) is a set of 2 dimensional panel objects created at the very start of the Active Worlds development.
There are several templates, each with standard textures:
- PD - 4x4x0 Door
- PW - 4x4x0 Window
- PP - 4x4x0 Flat Panel