Structural Truss families are quite easy to create: you sketch the top & bottom chords and the webs, and you’re pretty much done. You can obviously constrain the family so it can flex for different conditions, or build a static truss for a particular situation.
If you load structural framing families into it, you can assign them to the Top & Bottom chords and to Vertical and Diagonal webs. If you choose not to do this, you can change them in the type properties later inside the project. But in this case, what will Revit use when you sketch one in the project? The answer is: the last used structural framing family! So if you placed a concrete beam and then you place a truss with undefined members, you get this: