The low is some defects that may not part of the requirement, these defects may need more discussion, may change to change requests or enhancement. User can still process all flows.

Medium: affect the functionality but user has work around.

High: affect the functionality or block some other functionalities. Users can not process the flows because of these defects.

The low defects are some defects that function is working, but there is some little bit UI problem, for example spell errors, page layout.

The medium defects are the defects that important functional can be completed and good output when good data is input, but bad output when bad data is input.

The high defects are some defects that important functional can be completed, but bad output when good data is input.

The critical defects are some errors that make the system cannot run.