When you enable Is Submittable in a DocType is becomes a Submittable DocType. A Submittable doctype can have 3 states: Draft, Submitted and Cancelled. A document in the Draft state can be changed like any document, however once it is in Submitted state, the value of any field in the document cannot be changed. A Submitted document can be Cancelled, which makes the document invalid. If you notice, an extra field was added in our Library Membership doctype called Amended From. This field is used to keep track of amendments in documents. Once a document is Cancelled, it can only be amended, which means it can be duplicated and the cancelled document will be linked to the new amended document via the Amended From field.