HP48G soft menu toggle: How?

[DM48]

I want to write a program that has the option for SI or English units. Some soft menus on the 48G have a small square that will appear and disappear allowing one to do exactly what I want. To know which units should be used.

A good example of what I am talking about is the menu under the Equation Library (Right Shift 3). SI or ENGL may have the square depending on which the user has selected.

Anyone know how to use this feature? I am assuming I can use it with TMENU. Lots of assumptions on my part.

TIA