do I need to validate my SCIC-NRT RailPlus ticket (bought in Slovakia) in the machine before boarding a regional train in Italy?
No, validation is necessary only for FS regional tickets, which are valid for 6 hours since validation. In case of SCIC-NRT such 6 hour validity rule isn't even possible, so you can ask yourself at the ticket desk in Italy, but it's 99.9% that SCIC-NRT ticket can be used without machine validation
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