The Workspace class works off of information in the IDD; the ProposedEnergy+.idd for EnergyPlus objects and the OpenStudio.idd for OpenStudio objects. To answer your questions:

1. All the IddObject types are available, the one catch is that you have to replace ':' with '_' in the named. For example, to get all the 'SetpointManager:MultiZone:Humidity:Maximum' objects: workspace.getObjectsbyType("SetpointManager_MultiZone_Humidity_Maximum".to_IddObjectType)

2. The object's name, which is usually the first field for EnergyPlus objects and second field (after handle) for OpenStudio objects. In your example it is DOAS humidity max, !- Name

3. This finds objects which belong to a certain reference group in the Idd. For example, both the 'Zone' and 'BuildingSurface:Detailed' objects are tagged with\reference OutFaceEnvNames. This means that workspace.getObjectbyReference("OutFaceEnvNames") will return all 'Zone' and 'BuildingSurface:Detailed' objects (and any others in that reference group). The \reference tag is applied to the 'Name' field, other fields use the \object-list tag to indicate that field can refer to any object in the listed reference group. For example, the 'BuildingSurface:Detailed' field 'Outside Boundary Condition Object' is tagged with \object-list OutFaceEnvNames, that means it could point to a 'Zone' or 'BuildingSurface:Detailed' or any other object which has \reference OutFaceEnvNames on the 'Name' field. Does that make sense?