RT4k documentation has some more detail
https://consolemods.org/wiki/AV:RetroTINK-4K#SCART
RGBS is a form of RGB where sync is sent through a discrete line. The RetroTINK-4K accepts the following RGBS signal formats via SCART: Composite Sync (attenuated), Sync-on-Luma, and Sync-on-composite...