The policeman called for 'backup' [=called for other police to come and help him] because the suspect was carrying a gun.
There was a traffic 'backup' [=('Brit') 'tailback'] for miles on the road because of the accident.
His role on the team is to be a 'backup' to the regular quarterback.
(..) Officer down. Shots fired. I repeat, shots fired at Bo's Diner. Unit currently on scene, requesting backup. Possible female hostage. All units in the vicinity respond immediately... Who's this? It's Baby. (..)2017 Baby Driver