Routers are usually assigned a single NSAP address belonging to a single area. It is possible for up to three NSAP addresses to be configured provided all System IDs in each NSAP are the same but have different area identifiers.
A router with multiple NSAP addresses will maintain a single LSDB per level of prefixes it learns. This essentially merges all of its configured areas together in the LSDB. This allows flexibility when changing area numbers since you can maintain neighborship when bringing a new area number online and taking the old one down.
IS-Is areas normal operation depend on the routing level enabled on the routers. L1 routers in an area have no L2 LSDB and no information from other areas carried by L2 routers. L2 routers will create adjacencies with other L2 routers regardless of area ID and will share all information in their L2 LSDB.
Multiple areas in a domain are created usually for the purpose of address summarization, in Is-IS area summarization should be configured on each L1L2 router in the area. Area summarization is simply configured by the summary-address command inside the router isis section.