This validator allows the use of custom Jira expressions.
A couple examples:
Fields
Description (optional): The text that will appear in the conditions view.
Jira expression (required): The expression that the condition will evaluate against the transitioning issue.
Presets: Lets you load some preset expressions to use as a base.
Error message: The message that will be shown to the user if the configured expression fails.
Enabled: If the condition is disabled, the expression will always evaluate as true.
Useful external links
Atlassian documentation of Jira expressions