The uniting (aggregating, assigned, or realizing) concept (the “from” side of the relationship) is always an element; for assignment and realization it can be an element or a junction. The united (being aggregated, assigned to, or realized) concept (the “to” side of the relationship) may also be, in some cases, another relationship or junction.
Shouldn't this be moved to per relationship sections for the sake of readability? If not, I suggest to reformulate as saying that the uniting concept is ALWAYS an element and then providing two exceptions (over 3 relationships), is a bit odd. (JB)