Since all special versions of cityXplorer for nüvi 3790 have FID = 800 & PID = 1 you will only need one (1) unlock code for all of them. It seems not to be a widely spread knowledge that the "gmapsupp.unl" file can hold all unlock codes in a Garmin GPS unit. If one needs 100 unlock codes for some reason then all can be put inside "gmapsupp.unl".
Just as an example - If you have a memory card with Garmin Language Guide and 5 of the special cityXplorer maps, you just need one 25 character code for the language guide and one 25 character code for the 5 CX maps. All 2*25 = 50 characters written inside the "[Memory Card]:\Garmin\gmapsupp.unl" file as a consecutive string without delimiters or carriage return / line feed. The "gmapsupp.unl" file should be a multiple of 25 bytes (25, 50, 75 ... etc.).
If you don't put the unlock code in the "gmapsupp.unl" file then you have to make one UNL-file for each CX map or embed the unlock code inside the <CX>.img file. It would be easier to just use one unlock code and administrate that in the "gmapsupp.unl" file. This would allow adding more of the special CX maps to the Map directory without any extra copying of UNL data and/or UNL-files.
CX maps should be in the "Map" directory, either on the memory card or in the Garmin unit. The best source to find where a file belongs is to look inside "[GPS unit]:\Garmin\GarminDevice.xml".
This is the "GarminDevice.xml" part for CX maps and how it finds MMR data inside a CX img file.
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Here's just some example pictures with cityXplorer Melbourne (Sensis). It works great to simulate even when you're in Europe.
If you're not in the cityXplorer coverage area - Set the unit to GP simulation mode:
If you're not in the cityXplorer coverage area - Use the "Where To?" ...:
... and set a current location (simulated position):
Use the "Where To?" and choose a end point for your route. Then choose "Public Transit" for the route calculation:
As a result where should be a route with public transportation data: