People uses the radio commands to simply get help or to say orders to teammates, also, using radio commands is a part of tactics.
Cover me - When escorting hostages/V.I.P. or when planting/defusing a bomb
Taking fire - When you are being shot by the enemy even not being hit, use it.
Report in - You can use it to ask your teammates where is the enemy or if the place was clear (You can reply by team message because the Reporting in command is just useless).
Go go go - Assaulting the enemy, like attacking them.
Fall back - Telling your team to retreat, you can use this if:
-Your enemy is too many and strong
-When bomb is gonna explode just a few seconds.
Get in position - Telling your teammates to stay in position, if you are ready to attack press o+1, and to reply if you are in position press p+5.
She's gonna blow - When the bomb is gonna explode or an airstrike grenade was thrown
Need backup - When the enemy has more powerful than your team or if you have low hp you can use this.
Sector clear - When clearing a path, for example, there's an enemy on the hallway, after you killed them you can use this radio command to tell your teammates that the path is clear.
And many more, I didn't add the obvious radio commands and I'm too lazy to add the other
This is our lesson today thank you.