open means 'up to, but not including'
closed means 'up to and including'
example
x>3 would have open dot, meaning greater than 3, you can get close to 3, but not 3 like x=3.00000000000000000000001 is a solution, but you cant get to x=3
x>3 would be closed, this would include 3 and up so x=3 is a solution, als x=4, but not less than 3
