By setting an action or rule to nothing as the last of a set of chained tasks, it can be self-cancelling. This can be done as part of the rule/action itself and can help to avoid unwanted repetition/looping. It is more normally needed with rules than actions as rules run every agent update cycle.
Consider this rule:
When the rule is run, the note's $Color is set to 'blue' and the Rule is set to nothing. Thus the $Color assignment is run once-only before the rule ceases to exist. However, the above only sets the Rule to no value, it does not re-enable inheritance. For that a slightly different syntax is needed: