User Prefs color selection question

A question I’ve always intended to ask: in Prefs > User Settings, for Background Stroke color there are two boxes, but I can’t determine what the one in the right column does. What is it supposed to do?

It is the stroke color of other layers that are set to display in the Layers palette (with the eye symbol):

There should be a tooltip, I agree.

Aha! Would never have guessed. Thanks.