Omnichannel messaging coordinates one conversation across several channels, such as RCS, SMS, WhatsApp, email and push, so the customer sees one thread and the business one record.
Omnichannel versus multichannel
Multichannel means a business is present on several channels; omnichannel means those channels know about each other. The practical test is whether a customer who confirmed an appointment by tapping a button in RCS stops receiving the email reminder for the same appointment. If not, the channels are merely parallel.
Where RCS sits
RCS is unusual because it carries its own fallback. A message sent to a handset that cannot receive RCS becomes SMS or MMS automatically, from the same registered number, so RCS plus SMS behaves as one channel with two renderings rather than two channels. Compared with an app-based channel such as WhatsApp, RCS needs no install and no separate opt-in surface, which is why it tends to be the default mobile channel in a US omnichannel stack, with email for long-form content and push for customers already in the app.
The rule that keeps it compliant
Consent is per channel, not per customer. An email subscriber has not agreed to texts, and a STOP received by text must silence RCS and SMS together, since they share a sender.