einvoice-preflight now has a Serbia page, the 26th in its country set and the first outside the EU that is a clearance model already in force for ordinary B2B sales.
What vendor guides agree on: invoices between VAT-registered parties go through the state SEF platform as UBL 2.1, the buyer accepts or rejects each one there, and input VAT is deductible only on an accepted invoice. A PDF by email is not the invoice.
Where they disagree: the B2G and B2B start dates (most say B2B from 1 January 2023, one says 1 May 2024), the fine ranges, and whether the Serbian format is a strict EN 16931 profile. No Ministry of Finance or Tax Administration text was reachable, so the page says "vendor guides" throughout and points at efaktura.gov.rs.
The checker does not apply Serbian rules or SEF's validator. A clean result means the UBL passes the European rules it knows. A Serbian CIUS rule set is the obvious next slice if the official validator's rules can be read.