From Diablo Wiki
F2P is shorthand for "free to play" which is in reference, most often, to online games and MMOs. Free to play games simply cost the box price at retail, and then any playtime online after that is free, much to how Diablo II operates. However, many F2P MMOs have an online store of some sort where the player can buy vanity goods or, in some cases, upgrades to their gear, through RMT, which is often controversial.
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