Ever wanted to play a really really big game of Warhammer 40,000? This is the book you need then. Entire planets full of armies smashing into each other is easily done by using this system.
To put things in perspective, the last Apocalypse game I participated in had 13 players per side, and resided on a table that was 4 feet wide and 36 feet long, and every inch of it was covered in models. Hell one of the Titans was almost 3 feet tall and went in for 2500 points (and was summarily executed by my Blood Angels Chaplain's Chapter Relic).
Things get crazy in Apocalypse, so many armies need to bring more than the average detachment to battle. All of these and their special rules are included within, in addition to the actual gaming system.
This scan is cover to cover, but in black and white only. Newcomers make sure you try out a few games of normal sized 40k using the main rulebook I've posted under 5th Edition Rulebook before you dive into Apocalypse.