This gets into some technical programming stuff, but imagine that behind the scenes every node is basically a function (aka a “method” in object-oriented programming terms). They usually take some input data, do something with it, and then return an output. It’s also possible to have a method that executes code but doesn’t require any input. And it’s also possible to have a method that executes code but doesn’t return anything (called a “void” method).
So in this particular case, the backend for Dynamo for Civil 3D has a collection of “object descriptors” that populate that dropdown node of object types. Also on the backend, there is a method to add to the list of object descriptors, and so Paolo (the creator of the Civil 3D Toolkit package) created the DocumentExtensions.AddNewTypes node that essentially does that. It doesn’t require any input (it’s just adding new object descriptors) and it also doesn’t explicitly return anything (the result is a modified list).
You probably wouldn’t. Dynamo for Civil 3D is still in a growing stage, and even though many solutions on the forum utilize the Civil 3D Toolkit package, it’s important to remember that it’s basically an add-on/extension. Most users rely upon it at this point, but it isn’t part of the OOTB installation. It’s akin to downloading a plugin from the Autodesk App Store. A little more discussion here.