+The option C<default> can be quite a lot of things:
+
+=over 4
+
+=item 1. A scalar value. This is the value of the entry that's
+selected by default.
+
+=item 2. A hash reference for C<multiple=1>. Whether or not an entry
+is selected by default is looked up in this hash.
+
+=item 3. An array reference containing scalar values. Same as 1., just
+for the case of C<multiple=1>.
+
+=item 4. If C<default_value_key> is given: an array reference of hash
+references. For each hash reference the value belonging to the key
+C<default_value_key> is treated as one value to select by
+default. Constructs a hash that's treated like 3.
+
+=item 5. If C<default_value_key> is given: an array reference of
+blessed objects. For each object the value returne from calling the
+function named C<default_value_key> on the object is treated as one
+value to select by default. Constructs a hash that's treated like 3.
+
+=back
+
+5. also applies to single RDBO instances (due to 'wantarray'
+shenanigans assigning RDBO's relationships to a hash key will result
+in a single RDBO object being assigned instead of an array reference
+containing that single RDBO object).
+