In short
Application-to-Person (A2P) SMS is messaging generated by an application or business and delivered to a mobile subscriber. Common examples include OTPs, authentication messages, banking notifications, delivery updates, alerts and promotional messages. It is priced and regulated differently from person-to-person messaging.
What qualifies as A2P
The defining characteristic is the originator: a system rather than a person. The message is generated by an application, submitted over SMPP or HTTP by an enterprise or aggregator, and terminates on a subscriber's handset.
The categories differ substantially in value and urgency. One-time passwords and authentication codes are the most latency-sensitive and typically the highest-priced. Transactional messages — payment confirmations, delivery notifications, appointment reminders — are high volume and moderately time-sensitive. Promotional and marketing traffic is the least urgent and usually the most heavily regulated.
Why the distinction from P2P matters
P2P messaging is conversational, priced competitively or bundled, and largely unregulated for content. A2P is commercial, priced at termination rates negotiated between operators and aggregators, and subject to sender registration, content rules and consent requirements in most markets.
That gap in price is the entire economic motive for grey routing. If A2P terminates at a commercial rate and P2P interconnect is nearly free, injecting commercial traffic into the P2P path is worth doing, and the operator loses the difference on every message.
Classification is the control point
Because the commercial model follows the classification, the classifier is effectively part of the rate card. An SMS firewall that separates A2P from P2P, and OTP from promotional, is not only making security decisions — it is determining what the operator can invoice for.
This is why revenue-assurance teams have a direct interest in classifier accuracy, and why classification disputes with aggregators are commercial negotiations as much as technical ones.