I'll ask a mate who works there and get back to you.
Probably something boring along the lines of needing more people on the tills and is a call to get till qualified people to fill in until the rush dies down. They may take a few minute to finish whatever they are doing first.
OK, according to my mate, they only have 3 staff codes in the store she works in.
1 ... All available staff to checkout
10 ... First Aider needed
99 ... Suspected mystery shopper in store, be on best behavior to all customers
For shoplifters, they carry (and have at checkout) silent alarms that trigger a response from the security team to stand at the doors.