In poly, making a distinction between rules and boundaries is important, because it’s no longer about two people. Two people in a relationship with only each other can make rules with each other all day long as long as they both agree to them. In poly, when you create a rule between two people instead of stating your personal boundaries, the rule also affects any future third party but they didn’t have any say in it.*
Rules are declarations, whether agreed to or not, that you will enforce someone else’s behavior otherwise they will face consequences. Boundaries are clear definitions that you state about what you are comfortable with, and you act on your boundaries, instead of demanding that someone follow your rules. When it comes down to it, you are deciding whether you’re controlling someone else’s autonomy, rather than your own.