The entire workflow in Analysis Situs is built around the Part Node which is the data object
representing the CAD model in question. By convention, there is only one Part Node in the application.
Such a convention allows to reduce the number of arguments in the provided Tcl commands: if no object
is specified explicitly, it is always assumed that the command will apply to the Part Node. Basically,
Analysis Situs was designed as a Part analysis tool, so all other objects used in the software can be
seen as auxiliary or complementary entities (in contrast to Assembly design utilities which would
allow working with several Parts).