Condition on linked issues and Validation on linked issues are features aimed to set restrictions on allowed issue links, or to require existence of certain issue links.
Validation on linked issues checks up both, previously created issue links, and new issue links created in transition screen. When we want to check issue links created exclusively in transition screen, we should use Boolean validator with math, date-time or text-string terms with a boolean expression with function transitionLinkedIssues(). Example: Validating only issue links created In transition screen
Almost any restriction or requirement you can imagine based on issue links and on linked issues is possible using "Condition on linked issues" and "Validation on linked issues". You can use the following parameters to set your restriction or requirement: