by dougtempel
Initially, they recommend that you use the 30-card decks where all heroes are from the same sphere of influence. Once you get a few plays in and better understand how the game works, you'll want to start making your own decks where you:a) still use 3 heroes from one sphere;
b) use 2 heroes from one sphere, and a 3rd hero from a different sphere; or
c) use 3 heroes that are all from different spheres.
When you start making custom decks, there should be at least 50 cards in the deck with no more than three copies of a particular card (see p. 27 of the rulebook). It can be difficult to make good 50-card decks using just the cards from the core game, and I don't think it's necessary to do so if you don't want to (until you buy an expansion pack or two to get more card variety, you may want to build your own 30- or 40-card decks).
The point is that if you have heroes of mixed spheres in your deck, cards must be paid for using resources from a hero that matches the card's sphere (that's what they're showing on p. 13).