Many elements depend on their browsing context. For example, if you were trying to implement a Custom Element that behaves like IMG, the HTML spec opines:

"In a browsing context where scripting is enabled, user agents must obtain images immediately."

That means it is necessary to observe ownerDocument changes, since the ownerDocument is an element's link to its browsing context, and moving one of these elements to a new browsing context necessitates some immediate processing step (in this hypothetical case, obtaining images.)

It should be called ownerDocumentChangedCallback since ownerDocument is the name of the DOM property of the element. This callback should take two parameters, documents, the old document and new document.

In the case of moving an element from in the document tree of one browsing context to being in the document tree of another browsing context, the ordering of this callback relative to the entered/left view callbacks should be:

leftViewCallback of the old document
ownerDocumentChangedCallback old document to new document
enteredViewCallback of the new document