The trunk of "agreement" is a frame of heading and recitals (Frame) with sections (Sec) in the middle. The components are assembled into forms (Form) of various types.
Out0 (Out - zero) can be used (or referenced) in an outline file. It provides the socket for an outline, encourages consistency, and makes those files findable by a site-wide search for the term. E.g., click on Out0, then on "Used By". You will see all the outline files.