I find inherently problematic using the term “Secondary” to define or refer to relationships because of the hierarchical preference it insinuates. For me, the “Primary” is the person I feel most connected to, a stable person that I have a deep intimate and trusting relationship with and can return to as a source of support. (“the person I make the rules with”) but I prefer to have a mutual understanding of where things stand and a continuous dialogue as things change instead of calling them a “Primary”.
The use of terms like “secondary” or “tertiary” seem to insinuate a tier of preference in terms of fulfilling needs. I understand how the language can be useful to poly couples or families with more structured relationships (or hot in a D/s context), but I prefer to keep an open dialogue with all partners as our relationships and needs/desires evolve. This can be tricky because everyone’s understanding of and associations with terminology used may differ somewhat, but open discourse is essential to making any relationship work, poly or otherwise.