GL cost allocation?

Hello experts,

I have confusion regarding- regarding GL Expense Allocation-

at setup 1) Code -  no issue

             2) Basis - no issue

             3) Expense - in this we set account from which .....allocation takes place....OK.

But please Let me Know....is it necessary..same account will be placed in

                                                                 Setup-->SLA-->Cost Allocation-->Account Derivation Rule

and also tell me about priorities. and account used.

??

Thanks .

Kunal