When a user registered a LINE account, the user will have a LINE ID which they can share and allow others to add them by ID.
When a user add an Official Account as a friend or they send a message to an Official Account then LINE will assign an unique user ID to enable them to have send messages to each other in 1:1 chat. It is not possible for an Official Account to send messages to a LINE user using their LINE ID. When Conversation API receives a user's message, a Contact will be created and the unique key will be stored as Contact ID.
Learn more
