The upper bound of the dimension
This property constrains the size of the array. If it contains an integer, then the array must contain exactly that number of items (in the specified dimension) or the input will be rejected by the add-in function, with a message such as "Expected 3 cells for X".
If the property contains a name, then a variable of that name will be instantiated in the add-in functions, and the validation code will set it to the size of the array. If the variable has already been set, then it will be tested. If the size is different from the value of the variable, then the input will rejected, with a message such as "Expected 3 cells for X". By using the same bounding variable for two arrays, you can constrain multiple inputs to have the same size, width or height. See also Constraining the size of an argument in the User Guide.
If the property is left empty (the default) then there are no constraints on the size of the array.