For a long time, it was a pain to find out the referance plane of an X-Section. Until I got this by accident.
Notmally I suppress each plane and see the messages to see if the says about the section I was looking for...
There are two easy ways.
(1) Keep the mouse over the section of interest and the datum plane name will be shown at the bottom left corner of the base window.
(2) Keep the mouse over the section for few seconds and the same will be shown next to the mouse..
Easy, isn't it.. until you know it !!!