Withholding Tax function

Hi NAV Experts,

We are using NAV 2015 which already includes withholding tax function. Since we are going to run our business in Thailand and Indonesia, we need to implement withholding tax function. However, I can only find several introductions to that.
I have two questions:
1. Is WHT function applied to the service module?
I found WHT business/product posting group in the sales line and purchase line table, but cannot find them in the service line. Does it mean WHT is not workable in a service invoice?

2. Do you know any resource, documentation, or case study about WHT implementation? We are trying to implement on our own.

Thank you!