P2P, person-to-person messaging, is ordinary texting between two people from their own phones, as opposed to A2P traffic that software sends on behalf of a business.
Why carriers care about the difference
US carriers route and police the two kinds of traffic differently. P2P is conversational, human-paced and roughly balanced in both directions, so it flows from any ordinary mobile number without registration. A2P is one-to-many, automated and mostly outbound, and carriers expect it to come from a registered source: a 10DLC campaign, a verified toll-free number or a short code. The CTIA's Messaging Principles and Best Practices draw the same line between consumer and non-consumer traffic. Sending business volume from an unregistered "P2P" number is the classic way to get filtered or blocked.
P2P over RCS
Personal RCS chats are P2P: two people texting in Google Messages, or an iPhone and an Android phone talking over RCS since iOS 18.1. Google Messages encrypts those chats end to end between its own users. Business RCS is a different path entirely: it goes through a verified RBM agent, which is why a business cannot simply "turn on RCS" from a phone number.
On SimplyRCS
Every message SimplyRCS sends is A2P, from a verified RCS agent with a registered 10DLC or toll-free fallback, with the registration filed for you.