After planWeight is derived by the nonconditional calculation in the following figure, it can immediately be used elsewhere in this or subsequent Rulesheets. 

Note: Subsequent Rulesheets means Rulesheets executed later in a Ruleflow. The concept of a Ruleflow is discussed in the Quick Reference Guide.

An example of such usage appears in the following figure:

Figure 1. planWeight Derived and used in same Rulesheet

In Condition row a, planWeight is compared to the aircraft's grossWeight to make sure that the aircraft is not overloaded. An overloaded aircraft must not be allowed to fly, so the approved attribute is assigned a value of false.

This has the advantage of being both clear and easy to reuse—the term planWeight, once derived, can be used anywhere to represent the data produced by the calculation. It is also much simpler to use a single attribute in a rule expression than it is a long, complicated equation.

But, this does not mean that the equation cannot be modeled in a conditional expression, if preferred. The example shown in the following figure places the calculation in the LHS of the Conditional comparison to derive planWeight and compare it to grossWeight all in the same expression.

Figure 2. Calculation in a conditional expression

This approach might be preferable if the results of the calculation were not expected to be reused, or if adding an attribute like planWeight to the Vocabulary were not possible. Often, attributes like planWeight are very convenient intermediaries to carry calculated values that will be used in other rules in a Rulesheet. In cases where such attributes are conveniences, and are not used by external applications consuming a Rulesheet, they can be designated as transient attributes in the Vocabulary, which causes their icons to change from blue/yellow to orange/yellow.