Hi,
You don't need a KB article. It's right in the help file:
Subject to Withholding
If the document is subject to withholding, this option is marked and the default rate for the vendor appears. You can unmark this option until the document is posted.
You can subject documents to withholding on a document-by-document basis. You can mark only documents in the functional currency subject to withholding.
This option is available only if you entered a Withholding Vendor ID in the Company Setup Options window.
Hope this helps!
Kind regards,
Leslie