Hello Sumeet,
The thing is , when you create a table in ABAP data dictionary , there is a field (column) called with data type CLNT is created. which is used to check the login client.
For example : if you are logged in with client 100 , and the users with client 100 are not authorized to view the data created by Client 200, you'll not be able to view the data.
So According to my understanding session for any SAP log on is the instance you are log on to it. & cross client is something related to the login client you use for logging in to SAP system.