Hi,
Following is explanation for NUL & CUL (named and concurrent user license):
Named User license:
Typically associated with specific user names.
Allow 'physical users', that are named to access the system based, typically each 'physical users' will have their user name and password.
One named user license is occupied for each physical user accessing the BI Platform, typically this is one user account per 'physical' user.
A named user can have unlimited sessions if not restricted by CUL.
e.g you have 2 named user license. you created two users (say A & B). Once A is logged from a machine. Simultaneously, he can login from other computer as A itself. You would not be able to create more than 2 named users in this case.
Concurrent license:
Specify the number of sessions that can connect to BusinessObjects Enterprise at the same time.
Each physical user can consume more than one session, for example the same user could use both the browser on a PC and a mobile Application on a mobile device. This would create 2 sessions for the same physical user.
Flexible since a small concurrent license can support a large user base (provided all of them don't login simultaneously 
).
Read more, from following SAP Notes:
1285639 - What license types are available for Business Objects Enterprise?
Hope this information helps.
Regards,
Onkar