Should be the serial number of the GPS unit. You unlock your maps for use with your unit.
That means that, if you upload maps to your memory card and you insert that card into somebody else's StreetPilot, it will not work.
Only you can use the maps you payed for.
I have heard that you can use the maps for two units. If you own two units of course. I don't think the program will notice if the second unit belongs to your mate, but don't say I told you this ;-)