The Source Type refers to the frequency range of the instrument and affects how the pitch tracking, filters, and tone controls in certain algorithms will respond to the audio source.
If you are mostly using delay/modulation/reverbs with your synths, the source type setting will not make much of a difference. If you mostly use your synth for lead sounds, you can set it for Lead. If you mostly use it for sub bass sounds, you can set it to Sub.
https://cdn.eventideaudio.com/manuals/h90/1.7.5/content/system-menu/global.html
