Emphasis on the Word "Sub"

I went to lunch the other day and saw this sign on the door. I have no idea what criteria make a police sub station. There were no cells, no dispatchers, no forms. There wasn't even a single police office. So how does a location become a police sub station? And what is supposed to happen at one? Maybe every place ever visited by a police officer can claim to be a sub station. In that case, I officially live in the safest neighborhood in America.