Clarify `Exists` variables in docs
See
@@ -206,9 +206,13 @@ Exact
Exists
^^^^^^
-``Exists`` variables measure whether both, one, or neither of the fields are
-defined. This can be useful if the presence or absence of a field tells you
-something meaningful about the record.
+``Exists`` variables are useful if the presence or absence of a field tells you
+something meaningful about a pair of records. It differentiates between three
+different cases:
+
+ 1. The field is missing in both records.
+ 2. The field is missing in one of the records.
+ 3. The field is present in neither of the records.
.. code:: python
