I have been a ham for almost 20 years, and when on CW I have always sent my call, name, RST, and full QTH. I have noticed that now many operators send NR 5280 or something similar instead of their QTH. What does this number represent? I know for sure it is not a Grid Square.
I found a reasonable-looking answer here:
As @Mike Waters said, it's probably the sequence number for a contest, i.e. the number of contest stations they've worked.