An amount has more than two decimal places. 21 rules.
Every rule in this family says the same thing about a different field: this amount may not carry more than two decimal places.
Nobody types 12.49875 by hand, so it is almost always a computed value written out unrounded. A percentage discount, a currency conversion, or a floating point number serialised raw.
Round when you calculate the amount, not when you format it. Rounding on output leaves the printed figure disagreeing with the totals that were derived from the unrounded one, which trades a decimal rule for an arithmetic one.
Worth knowing before you over correct: the unit price is deliberately NOT capped at two decimals, because per unit pricing legitimately needs more.
Drop a Factur-X or ZUGFeRD file into the free validator and it names every rule that fails, with the explanation from this page attached to each one. The EN 16931 rules run in your browser, so the invoice is never uploaded. The ISO PDF/A-3, XRechnung and PEPPOL BIS checks need a server and are a separate opt in that says so.
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