All viruses, mercs, items and other monsters have different behaviours. (.bsc = behaviour script).
For viruses, it's the attack loop they are put in.
Mercs, attack loop.
Items, the skin it opens. az_bon for example is just a huge pile of bone structures for all surge parts. So the .bsc just chooses one of them and the skin it needs for the .bon.
Bubbleboy on Level 1 was not put into a script where he jumped attack alot, if they did, bubbleboy wouldn't be as "useful" or challenging. If you change the skin and model of him, he still gets the same loop. Sector .bsc can't change, nor merc ones either.