Follow the steps below in order to setup the Add Approval Summary on linked issues  Post Function on a workflow transition: 

• Choose   →  Issues → Workflows  
• Edit the concerned workflow
• Select the concerned workflow transition (transition on approbation or transition on rejection)
• Select the Post Function tab
• Add Post Function
• Check the option Add Approval Summary 
  
  
  

• Fill the parameters: 

  • User used to evaluate the applied criteria and to put the approval summary into a comment or into a custom field (text area multi line)  Required:  

    1. The specified Jql Filter will be evaluated  basing on the selected user. Ensure that the selected user has appropriate browsing permission on the projects at whom the current issue belong to.

    2. The Jira Comment will be added via the selected user credentials. 
    3. The Custom field will be edited via the selected user credentials. 
  • Customize the text of the approval log Optional:  Here you can customize the Approval Summary by adding an expression that follows the convention described below:
    
    1. add a text into the { } brackets
    2. add any admissible issue field inside the [ ] brackets. Admissible fields are: Approval Name, Action, Source Status, Transition, Approver, Destination Status, Date and Can Approve Detail
    3. concatenate each element via the &

E.g. {• Source Status: } &[Source Status]& {\\ } &{• Transition Name: }&[Transition]& {\\ } &{• "}&[Approval] &{" }&{:  } &[Action]&{ by }&[Approver]&{ on }&[Date] & {\\ } 

If this field is not filled, the default is applied. The default Approval Summary appears as follow:

• • JQL filter for current issue Optional: the post function will act only if the current issue satisfies the JQL filter. 

  • Add Summary on the selected custom field Optional: When filled, the Approval Summary will be put on the selected custom field.  Leave empty to add the Summary into a new Jira Comment of the current issue.

? On selecting a Text Field (multi-line) custom field, ensure to enable Wiki Style Renderering for this field

On setting a target custom fied, this field will be filled as follows:

 In order to allow the post function to work properly, keep in mind to keep it on the top (prior to Set issue status to the linked status of the destination workflow step).

 You can Add the Post-function on any transition.

 The Approval Summary is filled only if the Approval history of the issue is not empty.

 The Approval Summary is filled with last approval decision for each mapping applied on the issue identified by its Source status and transition on approbation.