If you are adding the ability to have more than one withholding tax in a single invoice. I assume that would mean applying a “Withholding tax code” here instead of specifying the percentage.
Using the same approach as existing tax codes, that would enable specifying multiple tax rates and associated accounts
If this is the case please consider instead implementing “Multiple rate” tax code as a set of single rate tax codes.
By which I mean, when creating a “Multiple rate” tax code, the user should add the constituent single rate tax code. The tax percentage and associated account being a property of the single rate tax code.
The reason is, this approach scales efficiently. Using his approach would also help existing tax codes if implemented there. More specifically it would help
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Limit the number of unique tax codes when different combinations of tax codes are used.
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Simply coding of custom reports / localisation as a leaf tax code only exists once, not as a component of multiple tax codes.
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Helps support regions where many tax codes in many combinations are used (such as USA).
PS:
Thinking about this more, if a sales invoice specific dollar value of with holding tax is required to be supported. Then that forces implementation of multiple tax code facility to be at the sales invoice not tax code level. Single rate withholding tax codes would still be appropriate (for name, associated account and percentage where used) but sales invoice specific dollar value would have to trigger display of a text box on the sales invoice screen to enter the amount. Unfortunately the resultant interface would not be as clean as just selecting a single (with potentially multiple account effects) withholding tax code at the sales invoice level.
It also means the description of an alternate “Multiple rate” tax code above is really only applicable for existing tax codes.