Unable to understand the system calculation of Withholding Tax when Threshold Control is enabled

Country: Puerto Rico

WHT Tax Rate: 10% (Supplier level exemption 6%)

Threshold Control Configuration:

Threshold Buckets Level: Party

Basis: Period

Calendar: Yearly

Threshold Type: Taxable Amount

Minimum Value: 500

Threshold Taxable Basis: Amount in excess of minimum value

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WHT Tax Rate: 7% (Supplier level exemption 6%)

Threshold Control Configuration:

Threshold Buckets Level: Party

Basis: Document

Threshold Type: Taxable Amount

Minimum Value: 1500

Threshold Taxable Basis: Amount in excess of minimum value

Can somebody help me to understand how system calculate the WHT tax for both types of withholding threshold