Each database residing on information servers can have unique characteristics. For example, databases may differ in the way they store data and in the access points available for searching. The records in each of the databases may also have different structures and consist of different data elements. However, in individual implementations, servers may be limited to specific formats of data that can be exported and the access points that are supported for searching. When a database is searched, the client passes a query to the server.
Queries can include different attribute types.
NISO Z39.50 enumerates these attribute types and their values in registered attribute sets. Standardized and mutually recognized attribute sets allow implementors a common basis for intersystem communication. After the server executes a search of a database, the server creates a result set consisting of those records that match the criteria of the query. Clients can request that servers return those records from a result set, or they can issue additional searches that further qualify a result set or use result sets as arguments in subsequent searches. These features are all in Version 2.