DIRSIG4 continues tracing until the path transmission falls below an internal threshold value. This is normally when maximum magnitude of the spectral transmission falls below 0.05 percent. Surfaces which fall beyond this threshold are not included in the output.
Internally, DIRSIG4 converts all input geometry files to meters. Thus while a scene may be specified in inches, the interactive mode will expect that the user-specified ray coordinates be given in meters. The easiest way to manage this issue is to build all DIRSIG scenes in meters.