The original edition of the game came with two large manuals - one a pilot's guide on how to play and the other a full colour guide to the Starsiege / Tribes universe. Both were around 200 pages long.
The multiplayer "chat" lobby is a actually regular IRC (internet relay chat) server. The IRC client is built into the game.
Starsiege is one of the few "mech" games that allows you to design your own "cammo" paint scheme. Just load up the file in a paint program and create your own!
The two campaigns, played from two different sides, have completely different endings. If you play as the humans, your final mission is to destroy Prometheus. If you play as the Cybrids, your final mission is to challenge Harabec Weathers, the finest HERC pilot Earth has, in a final duel.
The game came with a Compendium that chronicles the history of the Earthsiege/Starsiege universe. It's done in sort of a comic book style, and has some really nice artwork.