A2P (application-to-person)
The traffic class where a system, not a person, sends the SMS — one-time codes, delivery alerts and marketing that carriers price and filter separately from ordinary texting.
A2P (application-to-person) is the counterpart to P2P, person-to-person. The flow is almost always one-way: a server fires off the one-time code, the shipping notification or the campaign blast, and the recipient is rarely expected to reply. Volumes are huge and the traffic fans out to millions of numbers at once, which makes it easy to tell apart from two phones chatting.
Carriers charge per message and filter aggressively for spam, which is why an entire layer of middlemen — CPaaS providers like Twilio — sell deliverability as a service. In the US, A2P now flows through 10DLC registration; short codes and sender IDs cover other markets.