What are ground rules for creating issues?

The issue list was blank so I created this for several reasons:
1) so list would not be blank
2) to see if I had permission to create an issue
3) to understand the ground rules on creating issues.

Wrt #3:
3a) Issue 'Types' can be 'bug', 'task', 'improvement', 'new feature'. When is it appropriate to use which one?
3b) Priority can be 'blocker', 'critical', 'major', 'minor', 'trivial'. When is it appropriate to use which one?