Optionalapp_Required if using a channel that uses app-scoped channel identities. Currently, FB Messenger, Instagram, LINE, and WeChat use app-scoped channel identities, which means contacts will have different channel identities on different Conversation API apps. These can be thought of as virtual identities that are app-specific and, therefore, the app_id must be included in the API call.
The channel identity. This will differ from channel to channel. For example, a phone number for SMS, WhatsApp, and Viber Business.
A unique identity of message recipient on a particular channel. For example, the channel identity on SMS, WHATSAPP or VIBERBM is a MSISDN phone number.