Also called ISO Schematron, SCH
Schematron is an ISO standard for asserting rules about XML documents using XPath. EN 16931 publishes all 223 of its business rules as Schematron, which is why "does this invoice comply" has a definite answer rather than being a matter of interpretation.
Most specifications describe their rules in prose, and two implementers read the prose differently. EN 16931 ships the rules as executable assertions, so two validators running the official artefacts agree.
That is the reason our validator can run in your browser at all. The rules are data rather than code we wrote, so we compile the CEN artefacts and execute them client side instead of reimplementing 223 checks and hoping we matched.
A Schematron run emits SVRL, an XML report of which assertions failed. Each failure carries the rule id and the rule text, and that text is the specification's own wording.
This is the root of the error message problem. The output is precise, machine generated and written in the vocabulary of the spec, which is why we attach a plain language explanation to every rule rather than printing what the Schematron says and stopping.
The CEN artefacts compiled and run client side, so the invoice is never uploaded.
What each assertion means and what to change, for all 223.
