Off by default. Every address already arrives with the verification status the provider recorded when it indexed the address — valid, accept_all, webmail or unknown — and that costs you nothing extra.
Turn this on to re-check each address live instead, which adds SMTP-level detail (MX records, SMTP check, catch-all, disposable, gibberish) and a fresh per-address score. It is charged as its own event, once per address checked. Worth it before sending to a list that has been sitting for a while.
On an accept-all domain it cannot change the verdict. Those mail servers accept every address, so no live check can confirm one mailbox — the status stays accept_all and isVerified stays false, however high the score. You still get the SMTP detail and a per-address score that varies where the indexed confidence score does not, which is the only signal available for ranking contacts at such a domain. Decide with that in mind, because you are charged per address either way.