In a purchase order application, why would you choose to use a declare expression for calculating the order total?

Using a declare expression to calculate the order total in a purchase order application is particularly effective because it allows for the calculation of values that can be utilized across multiple forms. This means that whenever the components of the order total change—such as item quantities or prices—all forms that reference the declare expression will automatically reflect the updated total without needing to implement changes in each of those forms individually.

This approach not only promotes consistency across the application but also enhances maintainability. If there are changes in how order totals are computed, adjustments can be made in the declare expression itself, and these changes will propagate to all relevant forms seamlessly. The scalability this brings is beneficial, especially in larger applications where the same calculated value might be displayed in multiple locations.

The other options focus on different aspects, such as updating totals on individual forms or reducing the number of necessary rules, but they do not capture the primary advantage of a declare expression in allowing shared calculations across multiple user interfaces.