NAB stands for Name and Address book or Notes Address Book.
It is usually called names.nsf and is in the base directory of a server.
The NAB holds, user, group and server information including server replication information and mail in databases. If this is deleted from a server the server is usually broken!
NB: Each Notes client installation usually has a names.nsf database which is the personal address book.
This holds the user's connection documents, ie how they connect to each server, use pass through servers, different ids etc, location documents, which include different settings and home servers or proxies and sametime servers for different locations, and the user's address book holding extra email addresses and groups etc.
This is not a replica of the NAB and never should be! This uses a diffrerent template to a NAB.