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Side Quests are really important especially when you reach level 30. You should start doing side Quest as soon as you make your character and you get rewards like cash to repair broken gear and soda cola to use during tough missions. And when you get level 30 do every side Quest in sight to get marks of triumph towards your tier. This also gives you more experience points, which are important as well to get access to more missions. Side quest can be found near mission spots, in the headquarters and in mission instances. And that's all there is to it!