How do we control enclosure template to appear while performing state movement like Assign or Approval?

We can do this by going to setting page of particular process ‘Workflow management’->‘States’. In this page you need to click under ‘Customize fields’ column for a particular state name for which you want to display the enclosure template. You can control the visibility by clicking the ‘Allow Enclosure Template’ check box.

Please refer our online help link for more details - States