BPW is discussed on page 2 of the manual.

BPW is a variable that tracks whether you are working in Bent, Plan, or Wall mode. For example, when you are working in "Plan mode" and you draw an 8x12 Girt, it will have a dimension 8" across, and when extruded in "Plan mode" it will have a depth of 12".

It is critical that you set BPW when drawing timbers, dimensioning, labellig, and extruding. Read this section of the manual carefully. To see the current value of BPW type !bpw at the command prompt. To change quickly to the mode you want type bpw at the command prompt and press enter until the mode you want is set.