You will often want a calculation to update immediately whenever one of the attributes used by the calculations changes.
For example, if you have two numeric attributes on a window that are added together to give a third attribute, you will want the third attribute to update immediately if you change one of the other attributes:
if you change x or y, the x+y field updates immediately
To set up a calculation field that updates when you change its dependants:
In Object Designer, create the x, y, and x+y attributes.