One of the major functions of the Description Policy is an ability to create new Amazon Products (new ASIN(s)/ISBN(s)) on the basis of Magento products.
However, you can also create a Description Policy in case you are planning to add or update Product Information in Amazon Catalog. In other words, if you sell an item, which already exists in Amazon Catalog, and want to update or add some data, the Description Policy has to be created.
M2E Pro does not provide an opportunity to update absolutely all Product Information. Only a specific set of values can be updated. This is due to the technical restrictions of Amazon API. Product Types which are not currently supported for an update will be added in future.
Note: The creation of Description policy is not available for some Magento Products types (Simple Products with Custom Option or Bundle Products).