When constructing a compound wall with inserts, builders often want to know the dimensions of unfinished structural openings for the inserts. You can dimension a structural opening by choosing references on the outer boundary of the core layer in the wall.
The structural layer is typically within the core boundary of a wall. Inserts can be perpendicular or non-perpendicular to the core boundary.
To dimension to a core layer
(Detail Level)
Fine so that you can view the layers of the wall.
Dimension panel

Dimension references for perpendicular insert

Dimension references for non-perpendicular insert