If you create dimensioning of an assembly unit, you have to select Host Category to be included in Total measures. 

It tells the software to measure from its boundaries and keep dimensions outside of its bounding box.

In some situations, all elements are of the same Category, so if you will measure them with Include in Total Measure option ON, you will get dimensions snaping to the boundaries of all elements of this Category. In the example below you will see, that all elements here are created as Face Based elements and the position of the plate is measured from the boundaries of other elements from the same Category, which is wrong.

So, in this case, we can add specially named parameter Exclude from Total Measure and turn it ON. This will remove this element from the total dimension, although it will still measure the position of it.