It's quite simple & logical, really. Responding officers are esentailly assigned a job to do at the call. One officer is the contact officer--the person who'll actually talk to the subject. Depending on the nature of the call & its locale another may be assigned a less-lethal force (stun-bag, taser, etc.) and one, perhaps two will be assigned as a shooter. It's the shooters responsibility to decide when to shoot. He is the sole person(s) whose job it is to neutralize the threat with lethal force at his discretion within Dept. policy. This avoids the risk of cross-fire, sympathetic respones from other officers & allows the responding officers to use less-lethal force if practical.
For the officers here that may feel I'm giving away "trade secrets" this is something any individual can find on other websites & its descriprition still doesn't prevent it from being applied. If anything it informs the general public there is STILL lethal force out ther even if tasers are deployed. And for States that have a "right to stand" law, stopping a perp doesn't instantly allow you to use deadly force or even a weapon to stop the suspect. Keep in mind the force has to be REASONABLE, so pumping a few 12ga. slugs into your neighbor's kid 'cause he tried to siphon some gas is only gonna get you jail time.
This can get very complicated very quickly & iI suggest anyone wanting to know specifics about this check with local law enforcement and/or police related websites to get clarification on their specific question or State laws.