The _C_HLEVEL_ is the hierarchical level of the term. The term at the highest level of a hierarchy has a value of 0 and the next level has a value of 1 and so on.
The screen shots below show how the values in _C_HLEVEL_ determin the way ontology data looks in the user interface.

Example 1: C_HLEVELS 0 and 1

Example 2: C_HLEVELS 0, 1, and 2