An edit is essentially a rule - in terms of scope - but one which is run very occasionally (on setting and then roughly hourly thereafter). It is akin to an agent set to a low priority.
There is no priority control for edicts. They are designed for background. Housekeeping tasks that need to run occasionally but not often enough to have impact in the more pressing rules.
A note or agent's rule is stored in $Edict.
Edicts may be overridden locally in a note by using $EdictDisabled.