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 the assembly design utilities which would allow working with several parts).
Metadata is a piece of extra information which can be attached to the boundary elements of a part, such as its faces, edges or vertices. For example, to give face a dedicated color, you have to prepare a metadata object which will store the (r,g,b) components of that color. The metadata chunks are available in the corresponding branch of the Part Node. You have as many Nodes in this branch as many boundary elements are enriched with metadata.
Once the Part shape is modified by a modeling operator, the metadata should be actualized in accordance with the modification history (if available). The actualization is done within asiEngine_Part::Update() method.