Vendor Payment Apply and Vendor Bill Relationships

I have a question about Vendor Payment Apply Objects and Vendor Bills. Inside a Vendor Payment Apply Object is a "doc" property that identifies a Vendor Bill. Inside this Vendor Bill are a mix of VendorBillExpenses and VendorBillItems that represent the itemized bill. How are Vendor Payment Apply object totals matched to the Vendor Bill Items/Expenses? For Instance, If a Vendor Payment Apply object has a total of $3000 and has a Vendor Bill with 2 Vendor Bill Expense that equal $2000 and $3000 respectively, how do I know the amount allocated to each Vendor Bill Expense? Are all Vendor