Lets see if i can hit it. Basically, a conversion calculator built into that field popup. Maybe it only exists while the edit is displayed, and doesnt actually change units of the object or drawing document.

So, you clcik the circle's property and the edit size appears. Then clcik on the "in" and the unit options are displayed. By selecting one, the value is converted and can be copied. Closing the edit has it remain as it was in units. It's more like a built in conversion calculator. Seems limited to a few doing unit conversion work.