This function returns the name of the issue security level with the given id.
issueSecurityLevel(id) #Output: Text
You might want to use this function for several parser expression where you are e.g. checking against the current issue security level. So even if you rename the issue security level afterwards, the parser expression still works and won't break.
Imagine an expression like this:
%{issue.securityLevel} = issueSecurityLevel(10000)
instead of:
%{issue.securityLevel} = "View Issue"