What is mean for sensitivity when use the dog anti bark collar?

What is mean for sensitivity when use the dog anti bark collar?
When pet owner buys a dog anti bark collar, install the battery in,
Bark collar is automatically on 4th sensitivity level.

Why need to adjust the senstitivity level?
Some dogs bark with loud sound, which make bark collar is easy to trigged, so it needs to adjust the sentivity level lower, so bark collar can works well.

Some dogs bark with small volume, which makes bark collar is hard to trigged, so it nees to adjust the sensivity level higher, so bark collar can works well.