This language can be used in a manner that is descriptive, such as simply explaining the nature of a relationship ("I only see Stacy occasionally, she is my secondary significant other.") or prescriptive which dictates appropriate behavior in that relationship ("Tom is my primary partner and Stacy is my secondary significant other. So, I should put Tom's needs first!").
More About Secondary Significant Other
Terms like secondary significant other are not used by all polyamorous people. Some feel that this kind of language serves to designate one partner as "lesser" and creates a structure in which some people's needs get met and others do not.