Single opt-in is a one-step process where users submit their interest via a text to a number and they are immediately added to the contact list and can start receiving messages. It determines what steps are taken before a contact is subscribed to your product/service.

Double opt-in is a two-step process where users need to confirm their subscription via email as well. A potential customer fills out and submits your form/replies to your number. They will receive a confirmation SMS and click a link to verify the number and opt-in. Their contact details are then added to your contact list.