Clarification needed on WHT Posting Setup in Navision 5.0SP1

Dear Experts,
Could you please explain me on below questions
A) General Ledger Setup
“Min. WHT Calc only on Inv. Amt” - what is the usage of this field.
Manual Sales WHT Calc. - what is the usage of this field.

B) WHT Posting Setup
“WHT Calculation Rule” - how are the rules works?
“WHT Minimun Invoice Amount” - how this field works together with “WHT Calculation Rule”?

Does anyone has notes or guide for setting up WHT in Nav 5.0SP1?

Thank you very much