How to give a common name for example to a City areas made of more cells ? I tried giving a similar name but seems they have to be unique ...

Well the only place you see the name is when using a door to enter it, and i think it defaults to the worldspace name for exteriors.

Unfortunately, nobody seems to know how to make a worldspace without using a dozen or so other programs, even if the worldspace is supposed to just inherit the local Tamriel terrain.