A reaction is an emoji applied to a specific message, such as a thumbs-up or a heart, rather than a new message. RCS supports reactions; over SMS they arrive as a text like "Liked 'see you at 3'".
How reactions travel
In an RCS conversation a reaction is a small event tied to the original message's ID, so the receiving app renders it on that bubble. Google Messages has supported reactions for years, and iOS 18.1 brought Tapbacks over RCS, so an iPhone and an Android phone now exchange real reactions instead of the "Liked..." text that SMS and MMS produce.
Reactions in business messaging
There is no reaction message type in the RBM API, so a verified RBM agent does not send reactions. When a business wants a one-tap acknowledgement it uses a suggested reply such as "Got it" or "Confirm", which returns a structured event the business can act on. That is a better fit for business flows anyway: a reply button carries postback data, a thumbs-up does not.
Why the term matters for a sender
Reactions are a reminder that the customer's messaging app is a modern chat client. A message written for a 2010 SMS inbox, all caps, a shortened link and "reply Y", reads as spam in that context. Write RCS messages the way the rest of the thread looks.