Wing Chun has many "stop" kicks, thus named because they are used primarily to stop an advancing opponent.
There’s the Wing Chun side kick, with the toes in the natural "Pik Bo" fighting stance position:
|Bruce Lee's Side Kick|
Or the kick most commonly known simply as the "stop kick", which resembles a back kick but is executed with the front leg, with the toes turned toward the opponent's groin:
|Ip Man on Wooden Dummy|
The fighting stance stop kick, aka the side kick as demonstrated by Bruce Lee above, works best most of the time in realistic scenarios. In Wing Chun it is normally executed against the knee or groin, depending on the practitioner's height. (We never kick above our own waist level so as to avoid losing balance and speed.)
The turned toe stop kick is more effective if your opponent advances into you and you shift to the side into your Chum Kil stance, which places you behind your opponent. In that position, your foot is already turned with the toe to the side, so the kick is more natural and powerful.
Many students are initially confused about which to use and when to use it. However, as with all things, with practice the answer is usually quite apparent to those who understand Wing Chun's economy of motion principles.