Here are some examples of using filters for the email notification system. 

1) "message submitter" "is not" "current user". If subscribed to this filter, a user will get email notifications both when a new task is added and when some other user (not him/herself) adds a message (bug-note). 

2) "(task) handler" "is" "current user". If subscribed to this filter, a user will get email notifications only if he/she is the handler of the task. 

3) "(task) priority" "is" "high" and "message submitter" "is" "Customer". If subscribed to this filter, a user will get email notifications both when a high-priority task is added and when the Customer adds some messages to the task. 

4) "(task) handler" "is" "current user" and "message submitter" "is not" "current user". If subscribed to this filter, a user will get email notifications when other users add a message to the task the handler of which is the subscribed user. 

Subscribing to the All filter for a certain bug or task is similar to using the "watch" mode in some systems, i.e. a user will get email notifications whenever there is a change in the task status or any messages are added.