Important - You can configure the workflow transitions only if you have created a custom workflow. Follow the steps to create a custom workflow
What are status transitions?
The movement of a task from one status to another status is called a status transition. The number of statuses a task can move to, from a particular status is called possible status transitions.
What are workflow transition rules? how to set them up?
You can take control over how your tasks behave as they are progressing through the workflow by customizing transitions. These are workflow transition rules.
Here are the quick steps for you to set them for an existing or a new workflow that you are adding
STEP 1: Project Settings >> Workflow >> Click on the 
icon of the custom workflow (in this case “Windows OS upgrade workflow”) and select Edit
STEP 2: Click on the lightning bolt icon which represents the transition rules to be set
STEP 3A: Add all the necessary information here as to:
Who can execute the transition, the options here would be Anyone by default. If not you can create a custom group of project members by following the simple steps:
In order to add a custom group:
Step 3A.1) Project Settings >> Project Groups >> +New
Then in order to add members to that Group
Step 3A.2) Members >> Click on the 
icon to the extreme right of the member name which you want to add in that Project group and select Edit
Once you have a project group created, select them from the drop-down menu.
In this way, you can create a project member group, who can execute the transitions. By default, the option is set to “anyone”.
Step 3B: Select the conditions that need to be checked before a transition, suppose you want to mandatorily check the fields such as Start Date, End date and so on before status change, you can call out here.
STEP 3C) You can also add the actions to be performed after a transition happens
STEP 3D) When you click Add new action you can choose from the available post functions, and set any necessary parameters. These include
1. Update property
2. Email users in a group
3. Trigger webhook
4. Create subtask
5. Add tags
STEP-3D-1) Using a post function to update property
When a task moves from open to in progress, you can use the option of "set assignee as, set priority as" or add any other custom filed, it would automatically get updated post the transition.
STEP-3D-2) Using a post function to email users in a group
Email users in a group as an action using placeholders of a particular components like fields, priority, assignee etc in the subject and the body of the email.
STEP-3D-3) Using webhook trigger
Share information easily using trigger webhooks.
For example, if you want to push information on collaboration tools like Slack about the project updates using API, you can use this option.
STEP-3D-4) Create subtasks
With this transition, you can automatically break down a task into subtask and assign that small piece to an individual in your team on a trigger such as a status update.
STEP-3D-5) Add tags
When a task moves from one stage to another, add tags to it to sort them easily. This is mainly used for reporting purpose.
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