Like any gacha game, it's no surprise that Tower of Fantasy has a huge variety of different things to collect. But it doesn't just end at upgrade materials and currencies. Tower of Fantasy also has multiple different types of EXP to collect, each of which grants you levels in a different category. One of these is Bounty EXP, a type of experience you can earn by checking off one of your most valuable daily mission types.
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You can collect Bounty EXP by (what else?) completing Bounty Missions. Not every Bounty will grant the same amount of EXP either, so you may want to check the EXP reward at the bottom of the rewards list before deciding which Bounty to accept. As you gain more Bounty EXP, you'll eventually increase your Merc level, with the progress bar shown at the top of the Bounty Missions page shown above. Keep in mind that these missions reset on a daily basis, so new ones will always show up before too long. Before you can begin collecting any at all, though, you'll need to make sure you've even unlock Bounty Missions in the first place.
You don't start the game with the ability to take on Bounty Missions, but you will unlock them before the end of the game's first chapter. Specifically, you'll unlock them (along with a handful of other features) after completing the story mission where you turn off the Omnium Tower in Astra. Once you do, you'll speak with Franz for a moment to debrief, after which point Bounty Missions will unlock. Starting from the beginning of the game, this is likely to take you somewhere between two and four hours to complete.