Severity Levels : Types of Severity of Bug/Defect may be divided into four categories in software testing
S1 : This flaw implies that the process has been completely shut off, and no further action can be taken.
S2 : Major − This is a significant flaw that causes the system to fail. Certain elements of the system, however, are still operational.
S3 : Medium − It results in some unfavorable behavior, but the system remains functioning.
S4 : Low − It won't create any serious system failures.