Boundary operands should be use to constrain some parameters, such as the center thickness of the surface, to be greater or less than the specified target value. However, this boundaries can be violated if this results in a better overall performace (lower merit function). 

Therefore, if this boundary shouldn’t be violated, then the weight on the boundary constraint operand must be increased (>1), which will increase the “importance” of this operand, so that violating it will always make the merit function worse. 

More information about operand weights can be found browsing the help files such as:

The Optimize Tab (sequential ui mode) > Optimization Overview > Notes on Operand Weights