Channel selects the delivery channel, sms or email. The per-channel routes (/send/sms, /send/email) pin it, overriding whatever the body names; on the generic route it is required.
Sync must be exactly "true": delivery here is synchronous, and anything else answers 503 because the queue plane that would run an async dispatch is owned elsewhere. Over REST it rides as ?sync=true (the URL binds over the body); a by-name call states it in its arguments.
To is the destination address per recipient — a phone number for sms, an email address for email. Several recipients fan out into one provider call each, and the response shape follows the count (see the items field).