This was an issue that came up in my recent battle:
As I understand it, the shooting phase is resolved on a squad to squad basis. One squad declares their targets, then firing is resolved including all consequences, then the next squad declares targets and resolves their firing including all consequences etc.
But, I know of other gaming-rounds of old there would be other players who let all models declare their targets and then resolve the firing, similar to declare charges.
What is the difference, you may ask?
The first approach is much more efficient, since squads that fire after legal targets have already been destroyed or broken by fire of another squad can fire at other eligible targets,
whereas in the second approach two squads would fire simultaneously at a target even when the target is already destroyed or broken by the fire of one of them.
So how do you resolve the shooting phase?
"We do not destroy you because of what you are, but of what you might become."