The Golden Rule
Remember...
First and foremost, 2nd Edition 40k is meant to be fun. The rules should serve the story you want to tell on the tabletop, not dictate it. Remember, the ultimate goal is a memorable, immersive battle that leaves everyone eager for the next game. Be a good sport and have fun!
Warhammer 40,000 is a challenging and involving game, with many fantastic races, and endless possibilities. In a game of this size and level of complexity there are bound to be some situations where the rules seem unclear, or a particular situation lies outside the rules as they are written. This is inevitable, as we can't possibly give rules to cover every circumstance. Nor would we want to try, as that would restrict what you can and cannot do far too much. Players should feel free to invent and improvise, exploring the galaxy of Warhammer 40,000 for themselves and taking the game far beyond the published rules if they wish.
—Rick Priestley & Andy Chambers, Warhammer 40,000 Rulebook (2nd Edition)