I understand this is related to Basis, but just form the understanding from Functional side, I am presenting this simple explanation for Cross Client.
We all know that Cross Client Request impact all the Cilents in a particular server. Whenreate a new Object & we get a pop-up message that Table is Cross Client. Why does this happens?
Tables which are having MANDT field are called as Client dependent tables.
Tables which are NOT having MANDT field are called Client independent tables.
Go to T-Code SE16N & explore Table VBAK, VBRK, VBAP, KONV,etc.. & you will see that the First Field in these Tables is MANDT i.e. Client. These are Client Dependent Tables, means all the changes in these Tables will impact only a particular Client.
Now, Explore Table TTXID (Valid Taxt ID’s) & you will see that MANDT feild is not present in this Table. This means this Table is Cross Client & changes in this Table will impast all the Clients in the Server. As we all know that when we create a new Text Id, we get a Cross Client message. Same is true for Access Sequence.
Most of the Repository Tables in SAP are Cross-client. The changes made in ABAP Work-Bench are placed in Repository Tables so they are Cross-client.
Hope this helps,