We need to nest templates in the main template. The nested template should be able to display fields that the customer can fill. For example, there are two attachments to this message. The file 1.jpg: In this picture you can see how it displays a button with the plus sign. It has a red arrow. When I press the button it shows a nested template. In this template I can fill the event type and a path field, and nothing else. In our case we need to display fields that the user can fill, and the option of add more templates nested within the nested template. In the picture 2.jpg you can see the result that we need. This image 2 shows a red arrow that point to a green button. If you press the green button it displays another nested template. In this template you can fill in more fields. In addition you have another green button to nest another template.
Summarizing we need to create complex templates, that is, templates included in a main template.
If you mean RXT nesting by nested templates, WSO2 Governance Registry does not support nested RXTs as of now.