Describes a more detailed channel information of how the message was sent/received. Either the slug of a non-legacy channel or one of aircall,api,app,chat,chat-contact-form,chat-offline-capture,contact-form,email,facebook-comment,facebook-message,facebook-messenger,facebook-post,facebook-mention-post,facebook-mention-comment,facebook,facebook-review,facebook-review-comment,form,help-center,helpdesk,instagram-ad-comment,instagram-ad-media,instagram-comment,instagram-mention-comment,instagram-media,instagram-mention-media,instagram,instagram-direct-message,internal-note,ottspott-call,phone,rule,self_service,shopify,sms,system-message,twilio,yotpo-review,yotpo-review-public-comment,yotpo-review-private-comment,twitter-tweet,twitter-quoted-tweet,twitter-mention-tweet,twitter-direct-message,help-center-contact-form,whatsapp-message.