Why APRS runs on AX.25 but in connectionless mode: beacons are broadcast to everyone with no session set up and no acknowledgment expected. The frame type that carries data without a connection or sequence number is the Unnumbered Information (UI) frame, which is why APRS packets are often called "UI frames" and are addressed to a generic destination like APRS or BEACON.
Watch out Connect and Acknowledgement frames belong to normal connected-mode AX.25, where two stations establish a link and confirm each numbered packet; APRS deliberately skips that overhead so any station in range can copy the beacon.
APRS = Unconnected broadcast = UI frame. No connection, no ack, no sequence numbers.