Validators check that any input available to the transition (such as user-supplied input) is valid before the transition is performed. If a transition's validator 'fails', the transition's post functions will not be executed and the issue will not progress to the destination step of the transition.
You can use JJupin JJUPIN and SIL to define a large set of validators beside the default validators offered by JiraJIRA, by just adding a (k) SIL Validator to a transition and publishing the workflow, as described here.
Below are just a few common examples:
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