A boundry is a limit or edge that keeps you separate from another. A boundry is a limit that promotes integerity.
Boundries brings order to our lives. They let you be you and me be me. If we do not have boundries we need defences. If something outside yourself is making you angry,sad,or is hurting you this is a good place to start with developing a boundry. My boundry is about me feeling safe not about controling you. If someones stubstance abuse is a problem for you you can choose to throw them out of your life or you can choose to change the way you react to them. But, you cannot make another person change their behavior. If you say to someone, if you use again I am leaving and, they use and you don't leave then you do not have a boundry in place. If you choose to stop reacting to their use and go about your own life you have a boundry in place.
My boundry for my As is that I will not loan them money and they cannot come into my house and use drugs or come over high. I cannot tell them they cannot use because they will do what they want to do. I can control what goes on in my own space.
As Smoke says, "I hope I am I clear as mud"