Off duty is commonly used when doing events such as music festivals, big games etc a good one to watch is the Miami police dept Vlogs, they have a couple there around big events such as the ultra music festival and a nba playoffs game which gives you an insight into what they do (#6 or #8 is the music fest)
its nothing glamorous, traffic control and keeping the peace in the area and assisting securtiy personnel with their duties.
I believe they get coded off duty as the event organisers will pay for the police to be present at the event and not the taxpayers