I'll try my best to answer, but I'm also going to ask the other experts here on the forum to check my answer critically, to make sure I have not made any errors or omissions.
MapSource is an application that is used to transfer data to and from a GPSR, and to view data on a personal computer. The data can be comprised of map segments, waypoints, routes, and tracks. The map segments are created by the cartography company and cannot be modified. The waypoints and routes are created by the end user and can be modified at will. The tracks are created by the GPSR as you move around, and (generally speaking) cannot be modified.
The only intellectual property that MapSource has the capability to protect are the maps. Not all maps are protected (by way of a serial number unlock), only certain product lines such as City Select, City Navigator, and the BlueChart (marine) maps. Some maps are left unlocked intentionally, so they can be used on older GPSRs that don't support the unlock technology, or very simple, inexpensive GPRSs that Garmin didn't want to complicate too much by getting into unlock technology.
Anyone can load any (original version) of a locked map product onto any computer. But, until an unlock code has been entered, it is not possible to see all the detail of that map on the computer screen - it will just show major highways. Until the unlock code is entered, it is impossible to transfer any part of the cartography from the computer to the GPSR.
Once the unlock code has been entered, the full detail of the map can be seen on the computer. I think (and here is where I am not sure) that the map segments can be downloaded to any GPSR - regardless of serial number - BUT they will only be displayed on the GPSR that the unlock serial number corresponds to. In other words, the unlock serial number is matched to the GPSRs serial number. I think you could download a CN Europe mapset to someone else's 2610 CF card - and it would not work on that GPSR when you turned it on - but if you took the CF card out, and put it in your 2610 - that being the GPSR that the unlock code corresponds to - then when you turned your GPSR on, you would see the maps.
It is possible to have any number of unlock codes loaded into a computer. If you live in one town, and your friend lives in the other, you could both back up your individual unlock codes to a backup file, email that file to each other, load it in your computers, and voila - either one of you could use either computer to load either of the two GPSRs.
I'm not too up on how the serial numbers work, because when I am doing GPSR beta testing, all of the pre-production testing GPSRs that I am given have the same serial number. So, the only 'unique' unlock codes I have obtained on my own (by purchasing them) are for the GPSRs that I have bought myself - the last one of those being a SP III that I occasionally use just for comparison purposes.
Michael