What does the division into very low/low/medium/high clouds depend on?

The cloudiness classification is according to the following table (indicative values):

Very low - 1000 mb - from 0 to 111 m
Low - from 1000 mb to 800 mb - from 111 m to 1949 m
Medium - from 800 mb to 500 mb - from 19449 m to 5574 m
High - from 500 m to 150 m - from 5574 m to 13608 m

mb - pressure in millibars of standard atmosphere