Entities can be nested, to form a parent-child relationship. In nested relationship, entities can be nested the way you want using the Xpath node (e.g. /ParentCat/SubCat/SubSubCat/etc). For example, if adding a category called 'SciFi' under an existing category of 'Books', setting the Xpath node to <XPath>/Books/SciFi</XPath> will add the new category as a child of the old one.
On an update, if you want to set a field to NULL or "empty string" use an empty element syntax (e.g. <Summary>string</Summary>).
On an update, if you want to NOT TOUCH the existing value on that field, omit the field from the Xml input document.
Only name field is required. All other fields are optional. ML-encoded data can be included (<ml>....</ml>) to import values in multiple locales at once.
If the XPathLookup attribute is not null, the software will look for the matching entity name & type at that xpath, rather than the root level.
Remember that entities are tied to many other elements within the store. Be fully aware of the changes you're making before doing them.