Are you looking to customize documents so that they have different subforms ?
Please bear in mind that a Form is a way of looking at (or creating) a document, and the Form is not saved with the document. The document is just a collection of fields.
If you have documents, based on the same Form, that need to use different Sub Forms, then you would need to do the following...
- Include all of the Sub Forms in your main form.
- Have a Hide When field for each Sub Form's contents
- Allow the user to sellect which Sub Forms will be displayed on a document by document basis, by changing the Hide When fields.