Assessment Cost Element in Allocation Structure

Hello

Can anybody explain me the purpose of Assessment Cost Element in Allocation Structure? Which C.Element do we exactly put here? I understand the debit Cost element goes in Source and the credit cost element goes as settlement cost element, but not sure on Assessment C.Element.

Also if I am maintaning the Source Cost Elements in Source in Allocation structure, Is it necessary to mainatain seperate Source Structure?

Thanks

Sarang