I believe LR just explained why a designation such as "tertiary" would be different from a "secondary." Distance, amount of time spent, emotional investment, financial involvement, whatever - can all be factors in "grading" someone's involvement in one's life. Just ask yourself what reasons you would consider someone third place in your life. Seriously, though, you don't even need to use those terms. Not every poly tangle goes by hierarchical positioning. Many of us, myself included, find the whole primary/secondary/tertiary terminology rather icky. I think of people whom I see less often as "satellite relationships" rather than tertiaries.
Independent solo polyamorist seeking lover-friends willing to invest in friendship, companionship, and love, but without a need for partnership.
Never confuse commitment with exclusivity, love with ownership, nor sex with intimacy!
For me, it is far better to grasp the Universe as it really is than to persist in delusion, however satisfying and reassuring.
~ Carl Sagan