If a Yongnuo YN565ex flash can only be used remotely without triggers, as an optical slave,via menu options S1 & S2, what is the SL option for? I had hoped SL was to recieve wireless signals from a trigger, without the need for a receiver.
If you have the YN565EX version for Nikon, the SL mode allows it to receive signals from a Nikon Master flash using the automated iTTL flash exposure system. S1 & S2 modes are for manually set flash levels.
If you have the Canon version, SL mode similarly enables the YN565EX to work as a receiver with Canon's e-TTL automatic flash exposure system.
Both of these systems use light from the controlling flash to send exposure information; they are wireless, without the need for a separate trigger, but are not radio-based systems, and the YN-565 EX does not contain a radio receiver. Do note that (at least with the Canon e-TTL system) the controlling flash mounted on the camera can be set to only emit light to control the off camera slave just prior to the shutter opening, without actually adding any light to the frame during exposure.
SL is for the "smart" optical systems--Nikon's CLS or Canon's wireless eTTL systems. Depending on your camera body, you may have a master/commander for these systems in your pop-up flash, but for Nikon that requires a D7x00 body or higher, and in Canon's system, a T3i or later/higher body. The "EX" designation on Yongnuo flashes indicates that the flash is compatible with the Canon or Nikon (or sometimes both, as in the case of the YN-568EX) optical systems, which can transmit TTL, HSS, and remote power control, so they're quite a bit more functional than the S1/S2 "dumb" optical modes, but of course, suffer from the same limitations as all optical systems in terms of range and line of sight when used outdoors in bright light.
The YN-565EX does not have a radio slave built in. The only Yongnuo flashes that do are the YN-560III/YN-560IV (which are compatible with the YN-560-TX and RF-60x triggers), and the YN-600EX-RT (which is only compatible with Canon or Yongnuo's RT gear). It is a little confusing that only the Yongnuo bottom- and top- end radio triggers are integrated into their flashes, but not the mid-grade YN-622 triggers. All three of YN's radio triggering systems are 2.4 GHz, and are incompatible with each other.