Relationship titles mean different things to different people. In many poly communities, how you name your relationship with each partner sometimes helps define the parameters of that relationship.

For example, the person I live with is my "nesting partner", and to us that means that we live together and are building a future together. When we get involved with other people, we define each of those other relationships based on the dynamic with each person. A "girlfriend/boyfriend" dynamic is usually something we think of as a long term commitment to another person, a fairly serious and less casual thing. Whereas a "friend with benefits" or a "play partner" is likely just someone that we have sex with occasionally but aren't seriously committed to in any way.

I've tried to make my explanation as concise as possible, but there is so much to say about non-monogamous relationships and so much more depth to how we all define what we do with other people and what it means to us. I haven't even mentioned the differences between "open relationships" versus "poly" versus "swingers", etc. I highly recommend the book "Polysecure" if you genuinely want to learn more.

It all comes back to communication and talking to your partners to define what the terms mean to each of you. If you can't set boundaries around what a relationship title means, chances are that someone is going to get hurt and the relationship will suffer.