Hi Prasad,
SAP is providing 3 default clients.
000- Master Client-contains only customization data. This does not contain any master data or transactional data even in a non-IDES system.
001- Copy of master client- Replica of Client 000
066- Early Watch - This client is reserved for SAP to log into the system and fix issues
These 3 clients are the standard clients in SAP. These 3 clients are not supposed to be used in development,quality,production environments.
Client 000 is basically used as working client only when you do support pack upgrade or ABAP load generations (SGEN) and implementing additional languages, etc. Otherwise, client 000 should not be used as a working client. The same applies to client 001. But the only exception with 001 is, with Solution Manager, 001 will be your working client. You will do all configurations and obtain support from SAP through this client. With other Systems like BW and CRM, this client (001) will not be a working client. Two standard users (SAP* and DDIC) are defined in the clients 000 and 001.
The client 066 is used only for EarlyWatch functions (Monitoring and Performance).
EarlyWatch is delivered in client 066 and is protected using the password SUPPORT. This password needs to be changed for security purposes.
If you remember while logging into SAP, you have mentioned the client of 800 ( or any other 3 digit number ). Think of a client as a logical separator within an SAP instance. The name “Client” may seem a little weird for describing a method of logically separating a database. However, in the days of SAP R/2 when SAP used to run on a Mainframe, resources were pretty scarce and many customers hosted their applications on a single system. And each of these logical systems were called as a “Client”.
Hope this will clears ur doubt.
Cheers
Praveen