This commonly refers to the region of air that is directly contacting a solid surface.  This is the region where heat is transferred by convection and conduction.  Normal R-values remove such effects because they disappear as soon as one solid touches another.  In window coverings the area between the covering and the window is a significant contributor to the effective R-value. This effect, and that on the room-side of the test unit are both a part of the R-value of the covering.