This selects which Layer 3/4 protocol or application type, such as udp, tcp, ftp, http, or the RTP-specific voice/video/signaling values, this QoS rule matches, with the caveat that 'All' is only permitted on the default rule and RTP Signaling is not user-configurable. Choosing an enumerated value like rtpVoice rather than a generic port range gives voice traffic its own QoS treatment distinct from ordinary UDP, reflecting that this device has built-in awareness of RTP voice, video, and signaling as separate classes worth prioritizing apart from bulk protocols like ftp or http. For example, setting a rule's protocol to rtpVoice and giving it priority1 ensures phone calls get preferential treatment over a simultaneous http file download on the same link.