What is a validator?

A validator controls whether a transition can be successfully executed by the user. The validator is executed upon transitioning the issue. If the criteria of the validator are met, then the transition is executed. Otherwise, the issue will stay in its previous status and an error message will be displayed to the user.

Validators are executed after a possible transition screen. Thus, given a transition screen is present for a transition, the user input provided on that screen is taken into account.