Forward chaining updates a target value whenever one of the inputs to the expression changes. If the network of calculations is extensive enough, forward chaining may impact performance. If the calculated value doesn't need to change that frequently, backward chaining limits the impact.

So, if you wanted to sum the cost of the items on an order in real-time, forward chaining would be an appropriate option. However, if the total cost also depends on the shipping charge, tax, gift wrapping charge, or other fees that are determined later in the check-out process, then there is no value to constantly recalculating the total cost until the user provides information that can be used to determine those other charges.

Basically:

• If changes are infrequent, it doesn't really matter, so you can use forward chaining.
• If changes are frequent, and there is business value in staying current with the inputs, then use forward chaining.
• If the network is extensive and there is limited (or no) value in staying current with the changes, then use backward chaining.