Understand the extra step
Double opt-in commonly asks the person to confirm through a message sent to the submitted email address. Single opt-in does not use that additional confirmation step. Neither term explains every part of a platform’s consent or suppression handling.
Compare the right numbers
If your agreement requires confirmed subscriptions, unconfirmed submissions do not belong in the delivered-subscriber count. Review submission, confirmation and acceptance stages separately to understand where the journey is losing participants.
Agree the process before quoting
Tell the team which signup journey your business needs, what the platform supports and which event counts as an accepted subscriber. Requirements can differ by market and campaign; obtain appropriate review rather than choosing solely on the highest signup count.