Approval Validation on Linked Issues

Follow the steps below in order to setup the Approval Validation on Linked Issues validator on a workflow transition: 

1. Choose   →  Issues>Workflows  
2. Edit the concerned workflow
3. Select the concerned workflow transition
4. Select the Validators tab
5. Add Validator
6. Check the option for the Approval Validation on Linked Issues
   
   
7. Fill the parameters: 

   1. User used to evaluate the applied criteria ^_Required:  Any of the specified criteria will be evaluated  basing on the selected user. Ensure that the selected user has appropriate browsing permission on the projects at whom the current and the linked issues belong to.

   2. Issue Link Types ^Required: the check on passed Approval Conditions will be performed only on the issues linked to the current issue via the selected linked types

   3. Message to show when validation fails: A custom message (optional) to show when the validator returns false. When it's not specified, a default message appears: You cannot transition this issue because the Approval Validator on linked issues is not yet satisfied

   4. JQL filter for linked issues: the Approval Conditions will be checked only on the issues that satisfy the JQL filter

   5. JQL filter for current issue: the validator will be checked only if the current issue satisfies the JQL filter. 
      

 Have a look at this link to see an example: How to block a workflow transition on linked issues depending by parent issue Approval status.