When submitting a ticket via the web interface, you have the following choices available to you: Low, Medium, High and Urgent.
These should be used as follows:
The system is completely down. Multiple users are affected.
Business operations are severely impacted. This would include no email, major application not functioning for all or a significant section of users.
There is a significant impact on one or more users. Business functionality severely impacted.
Problem affecting one user or problem affecting many users but with low impact.
This denotes an irritation rather than a problem affecting normal work.