Most poly relationships have some form of this, whether it’s written down or not. Because there are no inherent rules or roles for poly, the parties involved have to talk about what they want, expect, need, demand, etc. Honesty, transparency, and on-going clear communication are cornerstones of ethical non-monogamy.
We had a really healthy and happy monogamous relationship for 20+ years, but when we opened up our relationship, we discovered how many assumptions, roles, lanes, expectations, etc. that we had been operating under. Even if you always plan to stay monogamous, this is a great process to not fall into those traps. I wish we’d done this at the very beginning of our relationship, although having navigated all that now, we’ve never been happier.