The section https://www.w3.org/TR/xpath-31/#id-unary-lookup defines the '?' unary lookup operator on maps and arrays, for arrays at the item (4.) it says about doing 

array?*

----- quote ----
If the KeySpecifier is a wildcard ("*"), the UnaryLookup operator is equivalent to the following expression:

for $k in 1 to array:size(.)
return .($k)

Note:

This is also equivalent to Section 17.3.17 array:flatten FO31.
---- end quote ----

The comparison to array:flatten seems wrong in my understanding as array:flatten is recursive while the array?* definition is not recursive.

So array:flatten([[1, 2], ['a', 'b']]) gives a sequence of atomic items (1, 2, "a", "b") while [[1, 2], ['a', 'b']]?* gives a sequence ([1, 2], ["a", "b"])
 of two arrays.

So I would like clarification on that note saying "This is also equivalent to Section 17.3.17 array:flatten FO31", in my view it is wrong.