...
The code above makes logical sense but it just won't work. In a nutshell this particularity of the implementation can be resumed this way:
The right hand side of an assignment expression resolves to the value of a a control passed to a rule when it is invoked. The left hand side uses a reference to the control'.
Lets work with a concrete example to understand the concept. Assume that there are two subtotals in the form, 12 and 14 and the total is correctly set to 26. Now, you add 50 as a third subtotal and the rule fires as a consequence. The values of the controls involved in the rule are passed as parameters to the rule: the subtotals 12,14 and 50 and the current total (value of T), 26. The core of the matter is in the expression:
...