How to define default client in SAP

You can set the SAP to default client login via Parameter.

"login/system client" value = specific Client(XXX)ex 100  

Every time when user going to login in SAP you will find 800 as client show in below.

It can be done using transaction RZ10. AS an example you can set the parameter value 800 in transaction code RZ10. A restart is needed  to make changes.