Guide Part 1
Part 1 of the guides represents the basics of the game, which are shared with a lot of other RTS. This part is manly useful to read for players that are new to the RTS genre. For all others it can be a nice addition in some parts, but overall you will not find a lot more things to learn.
Guide Part 2
Part 2 is slightly more advanced and is more directed to players who haven't played RTS games yet or have played RTS games that are different from IH in their core gameplay as AOE. This information will only partially be relevant for players who have played games similar to titles as COH.
Guide Part 3
Part 3 is information which is nice to know but not very important to play the game nor to understand its basics.