Slapping is an open hand strike that is less dangerous than real strikes such as punching and elbowing. This is because the palm has a larger surface area and is also softer than the knuckles. Slaps are also less rigid and will transfer less force. However, don’t underestimate slapping as there is a fine difference between a palm strike and a slap, and a palm strike can be as damaging as a punch.
In slap boxing, it’s not really boxing as you are limited to just wide looping slaps while in boxing, straights play a major role. Slapping is generally an act of degrading someone. Take for example Nate Diaz slapping Mcgregor multiple times in their bouts. It was a humiliation because Diaz did not see him as a big enough threat to not clown around at all.