After classification is the severity. These can be added with the plus icon, edit them using the standard edit icon, and then save using the standard save icon. 

Issue Severity  - enter a description of the severity of the issue. For example, High, Medium, Low.

Issue Severity Importance  - this is a scaled number which you can use to define the level of importance. How great a value must be to indicate a degree of severity is determined by you/ your organisations chosen scale. 

Issue Severity Full  - is where you can provide a description, which then would allow the person who raises the ticket to know which severity to select.