This is the original forum post:
PNM is a portable bitmap format used in legacy software. The use of PNM file format is more common in LINUX / UNIX Platform. PNM file format consists of three different file formats namely PBM, PGM and PPM. (Corrections suggested by Andrew Kirillov, thank you.)
PBM [Portable Bit Map - Binary]
PGM [Portable Grey Map - GreyScale]
PPM [Portable Pixel Map - Color]
Each of these file formats can be written in ASCII or Binary Encoding. The encoding is determined at reading time against the Identifier each PNM file contains called "Magic Number". This is always the first token of the PNM file.
"P1" [PBM - ASCII Encoding]
"P2" [PGM - ASCII Encoding]
"P3" [PPM - ASCII Encoding]
"P4" [PBM - Binary Encoding] (not yet implemented)
"P5" [PGM - Binary Encoding]
"P6" [PPM - Binary Encoding]
The original definitions of PGM and PNM do not describe 16-bit pixel formats. It has, however been used in many practical situations where accuracy/depth of color information matters. This plugin does not currently support the 16-bit extensions.