Available with Network Analyst license.
Network analysis classes are containers for network analysis objects. They can be tables or feature classes.
All network analysis classes, whether tables or feature classes, are listed in the Network Analyst window; however, only feature classes are listed in the ArcMap table of contents.
Network analysis classes can be input, output, or input/output classes.
Input classes provide information to the solver during the solve process. You create all the objects in input classes and set their field values before solving an analysis.
Output classes contain results only. While solving an analysis, the ArcGIS Network Analyst extension creates the objects in output classes and sets their field values to show results.
Input/Output classes are a combination of the other two classes. You create the objects for input/output classes and perhaps enter some field values before the solve process. Network Analyst updates field values to show results.