Three selectable ranges. Within each range, there are six automatically incrementing correction levels. On the first bark, your dog gets the lowest, mildest correction. The corrections gently increase with each successive bark until your dog finds the level that is sufficient to discourage it from barking. Some bark collars detect barking with a microphone and activate on sound, but can be falsely set off by other noises or another dogs bark.
Some bark collars use vibration but can be falsely set off by other motion or jostling.