As I understand it, IN
, NOT IN
, or BETWEEN
work essentially like the MySQL operators of the same name. They are just abstracted somewhat.
However, it sounds to me like you don’t need to worry about that. Your description makes me think that you are only worried about the meta_key
itself and not any particular meta_value
s or range of values, which means you can just use a simple meta_key
argument (Look for “Simple Custom Field Query:”)