Hold the 'Light' button lets u access the Slave mode where u can set to SL1 or SL2.
But when you press the mode button, there is other to modes. one is PP(the other 1 i cant remember. What uses those 2 modes are?
M = you manually set the level of flash power output
A = 'auto' thyristor mode, doesn't communicate with the camera like P-TTL does, instead you set the ISO and aperture and the flash adjusts power output based on the scene brightness
SB = only use the AF assist lamp on the flash.
IMHO A-mode is more reliable. P-TTL tend to meter under by 1-2 stops especially when the flash gets reflected somewhere in the scene. Also there is no pre-flashing in A-mode so you don't get shots with sleepy half-closed eyes among people whose eyes are more sensitive.