What is a second-level space boundary?

Second-level space boundary rules require that the floor or wall of a space be broken along the centerline of the floor or wall adjacent to it so as to correctly define the neighboring relations with adjacent spaces.

For example, look at the 1st floor floor-plan below. Notice spaces labelled “sp-3-Space” and “sp-2-Space”. “sp-3-Space” is 35′ long and “sp-2-Space” is 15′ long.

Now, look at the 2nd floor floor-plan below with “sp-6-Space” and “sp-5-Space”.

Notice that “sp-5-Space” is located directly above portions of both “sp-3-Space” and “sp-2-Space”. The figure above shows the internal wall that separates “sp-2-Space” and “sp-3-Space” on the first floor. This wall is represented with a dotted line. Second-level space boundary rules require that the floor of “sp-5-Space” be broken along the centerline of this wall to correctly define the neighboring relations between
“sp-5-Space” and “sp-2-Space” and “sp-3-Space below”.