If you reach the limit for your level you will level up and by doing so every time your maximum capacity of HP, CP and EP will rise giving you an advantage on the future fights to come.
You are limited to a certain level until you complete a grade change mission. You need to change your grade to advance above level 15, 30.
HP, CP and EP
HP, CP and EP are basically all hit points and determine how much damage you can take before losing a battle be it physical or electro-magnetical.
Stamina
Your stamina indicates the number of attacks you can make, as well as how strong those attacks are. The less stamina you have, the weaker your attacks.
It regenerates with time, you get 1 Stamina every 20 minutes.
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