Defining the Conditional Expression
Set up the basic comparison in the validation rule. On the Validation Execution tabbed page, enter the formula Sal!=null && Comm!=null in the Conditional Execution Expression text box.
This text box causes the rule to be applied only when the specified condition is true. Note that the expression is case-sensitive.
In the Triggering Attributes section, select Sal from the Available Attributes list, and click the Add (right-arrow) button to move Sal into the Selected Attribute list, along with Comm. At run time, when the value of any attribute in this list changes, the rule is reevaluated.