Common Mechanisms of UML

Flexible Severity Rather Than Absolutes

The combination of factors, of which number of bugs and their severity is treated as one component is used to measure the quality of a product
Many organisations have designed and used satisfactory, quantitative, quality metrics
Because bugs and their symptoms play a significant role in such metrics, as testing progresses, the quality rises to a reasonable value which is deemed to be safe to ship the product

The factors involved in bug severity are:

Correction Cost: The catastrophic bugs are corrected easily and small bugs may take major time to debug. So it is not as much important

Context and Application Dependency: Severity depends on the context and the application in which it is used

Creating Culture Dependency: 

What's important depends on the creators of software and their cultural aspirations

Test tool vendors are more sensitive about bugs in their software than games

User Culture Dependency: Severity also depends on user culture. Simple users of PC software go crazy over bugs, where as pros (experts) may just ignore

Software Development Phase: 

Severity depends on development phase

Any bug gets more severe as it gets closer to field use and more severe, the longer it has been around