[quote=@Gunther]
[@Thayr] Is knowledge of the rules of WH40K necessary? I do play WH-AOS. I own two Orruk armies: Kruleboyz and Ironjawz. I am interested in starting a WH40K army, but haven't yet. My curiosity in this genre is sufficient to motivate me to read and study the units you are talking about. I also happen to have 21 years personal experience in the US Army serving in the Infantry. So I do understand both real world tactics  and gaming tactics.
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I'd say that wikipedia knowledge - that is, what is a bolter, what does it do, qhat is a hive city, what is the state of the Imperium and language its citizens mind use - would generally be sufficient to play. Knowing the rules themselves for the actual tabletop is less needed.