In the grid component, in the row expander functionality window, there is an option to enter a show/hide expression on the tabs of the row expander.
What exactly does one put there?
The bit of documentation I could find mentions an example of using a session variable.
Is the session variable the only thing one can use?

I would like to use a field from the parent grid table and check it's contents to be like A or B something like

Horse = "Mare"

Where "Horse" is a field in the parent grid table.

However, this does not work. What exactly is the syntax / operands one can use in this show/hide expression?