We persist a document a1 containing an embedded document a1.1 which itself contains another embedded document a1.1.1. When the embedded document a1.1 is moved to another document of another collection b1 and needs to get persisted as embedded document b1.1 the embedded document a1.1.1 (which is now part of b1.1, too) is not saved as an embedded document of b1.1.
Example: We use a version object which contains a pageversion as an embedded document. This page object contains multiple elements (stored as embedded documents) which themselves contain several parameter objects (stored as embedded document of elements). Now assume we have such a persisted version and want to "copy" the pageversion in another collection. So we generate a new page object, and add to this page all elements of the persisted pageversion. If we now persist this page, the representation in mongodb contains all elements but not any parameters, which should be part of the page as well (as they are part of an element).