The information contained in the SAP systems, and thus in the database, can be classified in two groups:
Data that are client independent, and therefore valid for all clients
Data that are client dependent, and only available in the specific client
The client-dependent data are treated by the SAP system in their own special form. When a user is logged on in a SAP session in a particular client, the user is only able to see data that belong to that client or data that are client independent.
The way SAP internally implements this is by having the client field as a key field in the records that belong to tables that are client dependent.
An easy way to test this is by accessing directly with the database tools and doing an SQL select ...