The Real Time Streaming Protocol is a media control protocol that is used for controlling streaming audio and video streams. RTSP has a wide range of uses and is often leveraged by other media-related services such as SIP. It most commonly uses TCP and UDP port 554 but additional ports are used by the actual media controlled by RTSP.
FortiOS includes an RSTP session helper that opens the ports used by individual RTSP-controlled streams. FortiRecorder and FortiCamera use RTSP for video streaming.