Let's say you have Ember's "Fireball" mod, right? And you pick up another one. You can fuse the two of them together, and it gets stronger. You *could* fuse your Fireball with any other mod, but it gets the largest gain when fused when the same-name mod, ie, another Fireball. If you don't have another Fireball mod, fuse it with another mod of the same polarity, and it gains more that way (vs a mod with a different polarity).
~ The only difference between genius and stupidity is that genius has its limits. ~