The PLAYPAL lump is a collection of game palettes used for various purposes. In ZDoom, only the first is read, so if you are using a custom palette there is no need to have more than one. The format is a raw 256-color palette taking 768 bytes.
The remainder were used for various palette flash tints such as the red of taking damage, the white of picking up items, and the green of the radiation suit. ZDoom calculates all these in-game.