The tiers represent the various level of pre-order bonuses available while the game was in development.

Each tier is being sold separately, an only includes what's listed within them (meaning you need to buy all three to get everything).

Which is a little annoying, as they weren't super clear on that during the intro sale. So I bought the Ultimate Tier (which at the time said it included the Mega Tier), found out later that day as they fixed the listing that it *didn't* include the lower tiers, and then forgot to buy them during the sale while they were at a price I could stomach.