What is a red star cluster and how are they commonly used?

It is not a glorified firework! In a combat situation, a Red Star Cluster indicates an alarm the eneny is in the vicinity or has been spotted. It was also used to mark the enemy position. The green star cluster was to indicate a Patrol entering the wire to prevent friendly fire. I always carreied a green star cluster in my hand and readied when entering a NDP after spotting movement while on OP/LP. The star clusters were pretty much SOP in Vietnam. I spent damn near every night in Vietnam on either OP/LP or ambush while in the field. My team had been shot at twice while entering friendly position which stopped the firing. We were once engaed by a M2HB which thank God, the fire was over our heads and stopped once the cluster went off. Night time in Vietnam was extremely dangerious. Troops were nervious and the enemy took every advantage to bring fire. Returning to the perimeter was the most dangerious moments. There are no glorified firework used in combat. Smoke also has a purpose which is another subject. Training simulators can be classified as fireworks

It's a signaling device. It looks like a bright falling star. But when someone says, "sending up a red star cluster" they mean they're sending out a call for help from all available assets or personnel in the area. Basically, it's a call for help.

It's a glorified firework. They come in packaged tubes and the user smacks the bottom of on a hard surface which launches it. They're most commonly used as a signaling device due to visibility in day and night.

They come in different colors and called clusters since they typical branch off into 5 smaller glowing pieces after the device ignites in the air.