[Affects XQuery11 too.]

2 "Basics", para 4:

"[Definition: In certain situations a value is said to be 'undefined' (for example, the value of the context item, or the typed value of an element node). This term indicates that the property in question has no value and that any attempt to use its value results in an error.]"

We shouldn't say (as the first sentence does) that a *value* is undefined (an XDM instance can't be undefined), but rather (as the second does) that a "property* is undefined. (I.e., a box that normally contains a value does not currently contain anything.)