Which mode is normally used when sending RTTY signals via AFSK with an SSB transmitter?
Why Amateur RTTY grew out of mechanical teleprinters driving direct FSK, where the mark tone is higher in frequency than the space tone. When you feed audio tones into an SSB transmitter instead, lower sideband inverts the audio so the resulting RF mark and space land in the conventional (non-inverted) order. So the long-standing convention for AFSK RTTY on HF is lower sideband, and using upper sideband instead produces a reversed signal the other station cannot decode.
Watch out Upper sideband is the tempting pick because most other HF digital modes such as PSK31, FT8 and JS8 are run on USB, but RTTY is the classic exception. Double sideband and CW are not SSB transmit modes for feeding audio tones at all.
RTTY is the oddball: everything else digital is USB, RTTY AFSK is LSB. R for Reversed.