I never bothered with the details. But it was recently confirmed in EU ruling C-55/18 [1]. Plus the government sends someone to check my documentation every 4 years and then they'll calculate their expected employee salaries down to the penny and force me to issue refunds if that diverges from my company's calculation, so I know from experience that they are super strict about it.

[1] https://curia.europa.eu/juris/documents.jsf?num=C-55/18