Combat Squads is a set of home rules for gaming in the Warhammer 40K Universe. From the rulebook:
Combat Squads is an attempt at creating a set of competitive skirmish rules for Warhammer 40.000; a game set in the grim darkness of the far future.
Within this booklet you’ll find rules for constructing, and playing, with small Combat Squads using the miniatures you already have for your regular Warhammer 40.000 army.
As such, then you’ll need the core rulebook for Warhammer 40.000 (currently, the 5th edition rules) in order to play Combat Squads. Furthermore you’ll need this booklet, of course, and the codex for your chosen army.
The rules presented here in the following are intended to supplement your normal games of Warhammer 40.000 giving you a chance to play the odd one-off special mission during a campaign game. However, they have also been
written in such a fashion that you could play Combat Squads as is; with an inbuilt campaign system as well as a slew of new missions.