Alicorn is describing here a specific type of polyamorous relationship (classified on wikipedia as having Sub-relationships) . There are other types polyamory for example - "Group relationships, ... in which all consider themselves associated to one another, popularized to some extent by Robert A. Heinlein (in novels such as Stranger in a Strange Land, Time Enough for Love, Friday" -Triad :Three people romantically involved. (Commonly initiated by an established couple jointly dating a third person; however, there are many possible configurations.) -Quads : Relationships between a couple and another couple