It refers to the account that is going to be used on the allocation entry to close the dimension to be allocated (original dimension). When an account is assigned in this field, the account on the original dimension will not be closed. The account specified here will be the one to close the original dimension. It is thus possible to keep a trace of the allocated amounts. If this account is not assigned, the account to allocate is used to close the dimension. After the allocation, the accounts to be allocated will thus be fully closed (provided that the Allocation percentage is set to 100%).