SOI (Single Opt-In) and DOI (Double Opt-In) are models for confirming user subscriptions.
SOI (Single Opt-In) — the user leaves their data, such as email or phone number, and automatically becomes a subscriber, without requiring additional confirmation.
DOI (Double Opt-In) — after entering the data, the user must confirm their subscription, for example, by clicking on a link in an email. This model provides higher-quality and targeted leads, since the user confirms their desire to receive content or services with their action.