Every time you complete a mission, you will receive weapons as rewards. Some will have "seals" which can be common or uncommon attributes. There seems to be three levels of strength normal, +, ++.
Then you can use weapons to feed into the main one of your choice through fusion. I tried to level up a weapon with the high re-sell seal. Still same price. So I am now wondering if it is even worth it to level those up ir just sell them as soon as you earn them.