There's a simple guideline in Liferay's Jira wiki when creating an issue. Liferay may change the priority when processing an issue so this may not be 100% correct but may give you some guidelines.
http://www.liferay.com/community/wiki/-/wiki/Main/JIRA
Priority/Severity #
An issue has a severity level which indicates its importance. The currently defined severities are listed below.
•Critical - Crashes, loss of data, severe memory leak, security hole, no valid workaround.
•Major - Major loss of function.
•Minor - Minor loss of function, edge case not experienced by most users, or other problem where easy workaround is present.
•Trivial - Cosmetic problem like incorrect spelling or misaligned text.
Some examples:
•Causes data loss (e.g. account data) or data corruption on upgrade with no good workaround - Critical
•Causes Liferay to freeze after a particular operation with no good workaround - Critical
•Allows unauthenticated users to see protected content - Critical
•Crashes Liferay, workaround provided - Major
•Adding 10 document library portlets to a page causes javascript error - Minor
•Missing translation for phrase - Trivial
