Supplier bank account level assignment

Hi all

I want to know how Supplier bank account level like site and Address-OU are different from each otther. For example, if a bank account is assigned to either site level or at address-OU level, how these two cases are different. I am asking this as per my understanding, combination of site, address and OU is unique and assignment of bank account at site or address-OU should not  make any difference.

When I assigned two different bank accounts at site and address -OU level, bank account assigned at site level gets defaulted at invoice creaton. Is this standard functionality or can it be controlled?