Hello Cool
Sniffer mode is used to capture Layer 2 wireless frames and send them to a packet analyzer program such as Wireshark. In this mode, the AP will actively receive frames, and process them, and send them to the configured packet analyzer. There they can be saved into .pcap files (for Wireshark) for examination at a later time.
SE-Connect mode is different, in that it is used to perform spectrum analysis. The AP will “listen” to the RF band in the air and record the frequencies and wavelengths it “hears”. This is useful in discovering all of the sources of EM radiation within range, that may affect the performance of the wireless network. In this mode, no actual transmitted data is examined.
Strictly speaking, Sniffer mode functions in Layer 2 (Data Link) while SE-Connect functions in Layer 1 (Physical).
I hope this has been helpful! Stay healthy and safe!
Laz