The appearance of the default set can be controlled in the same way as other sets, for example, using an identifier of an empty string.
<set id="" appearance="panel" />This will render a panel around all the fields with a label of Default.
To specify a different label, add the label attribute. For example, the following label will be General.
<set id="" appearance="panel" label="General" />
You can also use a message bundle key.
<set id="" appearance="panel" label-id="form.set.general" />
Links:
[1] https://docs.alfresco.com/../concepts/forms-intro.html