I don't usually feel insecurity about the people I love. But when I have they can be broken down into categories
uncontrolled aspects of poly
uncontrolled aspects of a relationship structure
In my cases, insecurity comes about because of things I don't really have a say in.
Example 1 - we were with a girl and I knew it couldn't last. My insecurity was rampant because at any point she could just cut us off.
Example 2 - I feel insecurities about my emotions. I am scared to give parts of me over to people. I am worried they will see the real me and potentially run. I can't even connect dots on this one because I have never had this happen, it is something maybe I was born with. There are many things I am very very open with, but there are bits of me that only a few people know.
Example 3 - I feel insecurity based on my partners relationships and not having any control. Not that I want control but when I see something as bad, or know it will end badly. I don't have a say per se, but I can lay the information I see on the table. Thats a good thing but not actually having a say is painful sometimes. Veto power is what this relates to for me, and it can be used irresponsibly and in a hurtful manner which is why we don't have that rule on the table (amongst many others)
Insecurity also rears its ugly head in reverse. In order to protect myself my communication hits the shitter and I don't give enough information to my partner to make a decision together. Thereby controlling the situation by ignorance. This is something I am currently working on. Probably relates to number 2.
In all cases, I don't have a lot of rules (I can't think of any I have right now actually, besides safe sex, which shouldn't even really be considered a rule). I try my hardest to be open and communicate the good and the bad. Hopefully that helps keep everyone respectful and thoughtful.