Learn in the textbookRules for Handling Personal Information — Consent, Third-Party Provision, and Processing
DefinitionOpt-in (オプトイン) is a method where a person's information is collected and used, or notices are sent to them, only when they have clearly given consent in advance.
ExplanationThe purpose is explained first, the person is asked to take an action to agree, and only those who agree are included. People who stay silent or never reply are left out. A typical setup adds someone to a mailing list only after they tick the box themselves on a sign-up page.
AnalogyIt is like a dinner where seats are set only for guests who reply that they will come. No reply, no seat.
Memory hookTo opt in is to choose to step inside, and only the person can open that door.