With patience and care, the author can manually keep track of the changes made to a document. This is tedious and error-prone, but if carefully managed it has the advantage of providing the highest level of fidelity. This is the technique used to produce the Second Edition of XML 1.0. With an intelligent editing tool. An editing application with enough information about the authoring schema can track changes to a document.
Sophisticated applications can maintain a complete history of all changes.
Although the author generally loses the ability to specify the level of granularity for individual changes, the tremendous benefit of having the authoring tool keep track of the changes generally outweighs this small sacrifice. Unfortunately, these applications are uncommon and often expensive. And because they require information about the schema, they are not easily portable across different authoring schemas. By automated comparison of two documents. This is the approach that diffmk takes.