"Mandatory" custom field

Hi all,

I am little confused about RT’s interpretation of “Mandatory” as
it relates to Custom Fields.

I have a CF’s validation set to the canned regexp (?#Mandatory)

The CF in my test ticket has pointedly been left with no value.

Aside from a meager red warning on the page indicating it’s not
set properly, all other actions on the ticket work just fine. I
can even resolve the ticket while the CF is unset ("(no value)")

So Mandatory how? When does that apply exactly? Does it simply
mean, as I seem to be finding, “do whatever you want anyway,
but we’re going to put red text near your offending field” ?