what is purpose of two Procurement BU approval attributes in Supplier Spend Authorization Approvals

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what is purpose of two Procurement BU approval attributes in Supplier Spend Authorization Approvals

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in "manage supplier spend authorization approvals" i observed that there are two 'Procurement BU' attributes. one for supplier header dimension and other for supplier site dimension. can someone please explain how these two function.

im registering an external prospective supplier for UN GTQ BU with only Company Details and Contact(without any address)..and using Procurement BU supplier header dimension attribute for spend auth validation in my approval rule. if i promote this supplier to spend authorization, system is rejecting the request. But the same request is getting approved if i add address details during registration and use Procurement BU supplier site dimension attribute in approval rules.\\