A couple of different game mechanics pages:
As you play the game you will unlock new attachments to customize your ship. Some of those attachments will have a carrot '^' on the top right of it's name. This means as you use this item you have a chance of unlocking an improved version of that item. Usually the order of items starts with Prototype (Proto) then Normal, then Adjusted (Adj), and lastly Improved (Imp).
As you unlock attachments in the game some will have a small 'X' on the top right of the items name. This means the attachment can be combined with other attachments to unlock new ones. For example the Super Spread and the Linked Blaster can be combined into the Spread Linked Blaster So you can get the benefit of both weapons but only have to use one slot.
Many of these combinations can't be upgraded further, but the individual attachments can still be upgraded to improved versions.
Stun is a stat that all weapons have. Attachments can increase this stat to increase the time the automaton is stunned when hit by that weapon. This can be tested in the Staging Area in the Garage.
Enemies also have a stun resistance stat, and generally the larger the enemy, the more resistant it is to stun.