IT:AD:CCC

Card (or often a Post-it note) is a physical token giving tangible and durable form to what would otherwise only be an abstraction;

Conversation is taking place at different time and places during a project between the various stakeholders concerned by the given feature (customers, users, developers, testers, etc.), which is largely verbal but most often supplemented by documentation;

Confirmation, the more formal the better, ensures that the objectives the conversation revolved around have been reached finally.