There are certain tables, in SAP, which when changed will not affect similar tables in other Clients. These are known as Client-Dependent tables. All Client-dependent tables have MANDT as their first field.
On the other hand, if a change made in one Client is reflected in another table across various Clients, then such a table is called Client-Independent. In this case, the first field of the table will not be MANDT. You need to be extra careful when changing the settings or content of these tables as this will affect all the Clients. It mostly happens with the customizing tables. when you try to configure some customizing changes and the SAP displays a pop up saying that the table is cross client. Be careful, because the changes will impact all the clients in the system.