There are four types of severity ratings as shown in the table:
Severity 1 (showstoppers): These kinds of defects do not allow the application to move ahead. So they are also called showstopper defects.
Severity 2 (application continues with severe defects): Application continues working with these types of defects, but they can have high implications, later, which can be more difficult to remove.
Severity 3 (application continues with one, expected results): In this scenario the application continues but with unexpected results.
Severity 4 (suggestions): Defects with these severities are suggestions given by the customer to make the application better. These kinds of defects have the least priority and are considered at the end of the project or during the maintenance stage of the project.