OK, we have the concept of a SAP instance, and that instance has a database which contains thousands of tables which contain a whole bunch of rows.  After SAP is installed, these tables need to have some base data in order for customization and configuration to begin.  Like state abbreviations, country codes, HR titles, etc.

SAP provides a subset of this data so that the Basis team can get in the new instance, add themselves a user ID in client 000 – “our” client - and start the real work.  We don’t want to mess this subset of data up so we need to populate it to a “work” place for the Functional Team to do their work.  Or several work places.

The base SAP instance comes with two clients:  000 and 066.  Forget client 066, it is used by SAP when you get close to GoLive and you want an EarlyWatch report.  This is optional and I believe a fee is involved for this service.

All the base or subset data is contained in client 000.  Also, client 000 is where the Basis Team does a lot of its maintenance like patching.  The Basis Team people are the only implementation members who will ever have access to client 000.  You can think of client 000 as the owner of all the client independent data in the SAP instance. Explanation in a minute.