Concurrent licensing allows the licensing of a number of copies to be simultaneously open inside an IP range (typically). What this means is that any time 10 (say) people can open a book simultaneously, anywhere inside the IP range, but if an 11th tries they are refused because ‘all the copies are out.’
When somebody closes a copy it frees the copy so that anyone else can now open it, until they run out of copies that are not in use.
It is different from a roaming system, which normally allows fixed users to access document from anywhere in the IP range (or domain).
The only way this can be achieved is using Safeguard Portable for USB, assuming of course that you wish to issue physical tokens.