I have images for this question, but I don't know how to attach them: there doesn't seem to be an option...

I am working on a database for a local college: I am cataloguing all of their buildings on campus; part of that includes cataloguing each of the classrooms within each of the buildings.  For the purposes of Master Planning, each room within a building has an FAE Category Code that explains what the purpose or use of the room is.  On the Building Form, I want to have a subform, where I can add each classroom within that particular building (which is then stored in a MasterClassroomT for later use): user adds the classroom number --> from a drop-down menu, select the FAE Code --> with the code selected, the FAE Category is automatically filled in --> and then the user can fill in the remaining info (square footage of the room, number of seats in the room, and any notes they may have).

I know that this can be done, and I am fairly certain that I saw it in one of your other videos; however, my wires are crossed, and the dots are connecting.  Can you help me out, sir? What step did I miss?  Are my relationships not set up correctly?  Or is it a property that I have overlooked?