I'm assuming you are using an old version (2020 R1 or earlier) and you are referring to this in the XactMeasure dialog?
It is for specifying a reduced material boundary size and is very rarely needed. You should only enter a value if there is a reduce material boundary specified on your drawing, in which case it would override the natural material boundary. I have only ever seen a handful of drawings that specify a boundary size so in 99% of cases, that field should be blank.
