From the preface:
This guide is designed to assist people in the development of a complete mythos; that is, a fantasy universe. The focus here is on natural products and processes. The guide does not attempt to address sociopolitical, technoindustrial, or religious concerns. However, following the instructions herein can provide the environment in which societies, and their associated phenomena, develop.
The guide is referred to as both the "Planetary" and the "Planet" Engineer's Guide. The guide itself constitutes the only significant change to the rules between the 3rd and 4th Editions of the game (the Player's Handbook received a minor revision). The guide was released in RTF and DOC format; both are identical except in very minor formatting. The DOC version was released as a self-extracting archive (EXE).