There is no single feed of every vehicle. To find them, the collector polls the live departure board of hundreds of stations, one after another. One full pass over all of them is a sweep.

#N is the sweep counter since the server started, stn is how many stations the last sweep queried, and the seconds are how long it took. A new sweep starts as soon as the previous one ends, so this is roughly how old the slowest vehicle position on the map can be.