Is it just FFA or are there team modes?
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There are quite a few modes.
Free Roam is the default mode with the most to it. You can join or create parties, also known as posses, and do gang hideouts together, take on other players, or just kill as many lawmen as you can, often while holed up in a town or a fort. There are free for all matches, gang matches (team vs team) and many other types of matches. There are now co op matches available too, which you can do with a team. It's free dlc. Then advanced co op missions on top of that. In addition to all of this, it is still rpg like. Everyone has their own character and can pick from a list of looks. There are 50 default levels to gain, which you unlock new mounts, character looks, weapons, challenges, game types, titles, and more. You gain XP by killing other players, beating gang hideouts, killing npc's, completing challenges, doing co op missions, etc. You gain more xp by working with others than you do by killing them. You are still able to pick up all the weapons in the game during gameplay, as well as snatch other mounts and other players' mounts, even if you do not have them unlocked. Weapons you find stay with you throughout a session. You always have weapons you've unlocked, but weapons you simply find only last until you quit the current session. (you could leave one free roam game and go to another and still have them, but logging off and back on gets rid of them) As for challenges, there are many of various types. Challenges give you extra xp for doing what it describes. For instance, a stat challenge known as Lemming, required you to fall to your death a certain number of times. There are co op challenges, pvp challenges, stat challenges, general free roam challenges, etc. There are also challenges like Master Hunter II, which at rank 1 requires you to kill five grizzly bears, one of which must be done with a melee knife. Also, after you get to level 50, you have 50 more levels you can gain to unlock a sweet mount and a legendary character, after that, 50 more for the same reward, and after that, 50 more.
There's a lot more to it than what I described, but trust me it's fun.