National VAT registers behind the fallback
VIES is a gateway to 27 national tax authorities, so when one of them goes down VIES has nothing to say. For 10 EU countries the same authority publishes its VAT register directly, and on the Pro and Business plans Avatcado asks it instead of failing the request. 6 of those registers answer per number and 4 are mirrored from the files their authority publishes. Each page below covers one of them.
When a register answers instead of VIES
Only on an outage. A validation reaches a national register after the VIES call has been retried and has still come back with no answer, and a number VIES does answer for never touches one, whether that answer is valid or invalid. A register can also only ever confirm a registration: a miss is never turned into valid: false, because the published registers do not cover everything VIES covers. The national registry fallback guide walks through the precedence order and the response fields, and the country coverage page lists what backs every country Avatcado validates.
A fallback answer is an ordinary validation response with meta.source_status set to "fallback" and meta.source naming the register, so your code can tell the two apart in one field. Get an API key or read the API documentation.