Fusions level in the book are the MINIMUM level of weapon they can fit in and the amount of levels it occupy in said weapon. A lvl 4 weapon can fit a lvl 1 fusion and a lvl 3 fusion. The total fusion levels combined can not exceed the weapon level.
Fusions cost per weapon, they all cost based on the weapon level. To install a lvl 5 fusion in a lvl 5 weapon costs the same as installing a lvl 1 fusion in the same weapon. Consequently it costs twice as much to put 2 fusions in a weapon.
Fusions can be moved by your mechanic or technomancer for half the cost of the target weapon.
So moving that lvl 1 and lvl 3 fusion from your Squad Machine Gun to a new nice lvl 8 boomstick costs half as much as getting two new fusions. (1150 per fusion instead of 2300 as normal). This is a bigger saving than the original 680 per fusion you payed to install the fusion in the machine gun to start with.
To get around this, there is an alternative called Fusion Seals they cost 10% extra but are essentially a removable fusion that you can swap between weapons. Their level is the maximum item level to use the fusion on.
A Disruptive Fusion Seal lvl 8 costs 2530 (up from 2300) but can be moved (every 24 hours) between weapons of level 8 or lower without a cost. That way you can swap between a laser rifle and a cannon depending on what you think the next day's fighting will be about. (I.e you know demons are resistant to fire so you use your backup rifle on this planet and go back to the laser when you return to the ship).
