Hello,
Italian setup does not deviate from standard Withholding tax setup in IFS. 
You need to define tax codes with Withholding Tax tax type and required percentages, and corresponding Income types. 
Then you need to enable Withholding tax reporting for supplier (Invoice info) - and under RMB connect Income type and tax code to the supplier.

Once done, all you need is to make sure income type is indicated on supplier invoice line. 
Please pay attention to company parameter “Post Preliminary tax Withholding” - if it is checked, withholding tax is deducted from invoice amount at invoice entry (by U voucher) so invoice is presented in GL in payable amount only and at payment to supplier WHT tax is converted from deducted to due. Otherwise tax is deducted at payment only.

Hello,

Thank you for your response. How can these withholding tax items be reported? Is any specific setup required, or is there an interface in version 8 where you can directly query what is still payable? How can I set a limit below which no withholding tax is applied?

Thank you!

Regards, Ili

Hello,
Regarding limit, I am not sure, in my case I have following income types (without threshold amounts):

I think business will simply leave income type empty on invoice line with WHT non-applicable. 

Regarding reporting, I can see just generic monthly tax report based on template 21 (tax sales detailed) with all WHT tax codes connected. 
I am sorry, but Italian company I was supporting is no longer active in IFS system, so I have no one to ask.