When a person must choose to not do something, otherwise it will happen automatically.

Opt-in describes the default state in any situation in which a user or participant has a choice of whether to do something or not, and the default state prior to any user involvement or active decision is “doing it”.

That is, unless the user consciously and deliberately decides to not agree, or participate, and chooses "no", the assumption going forward is that he or she agrees to the conditions proposed.