A closed dot is used when it doesn't include that value. For example, if you have "4 > x" we would use an open dot at 4 because 4 can not be included since it is greater than or equal to. We use a close dot when something is greater. lesser, or equal to somthing. Like "4 > x." Basically. you use a closed dot when it's just less than, greater than, or equal to, and open dot when it's just less than or greater than.