As follows from above, each value field is a "small expression input line". The difference is that new identifiers are not allowed in the value fields. The expressions there must use only parameter names already present in the main expression.
Note: MathMate has a special "meta - variable" X which is automatically assigned the value of the last computed expression. It may be used for iteration or just for convenience of computations, allowing to break complicated expression into pieces. If you include the X identifier in your expression, don't be surprised if the next computation of the same expression returns a different value. This is because X has changed its value after the first computation.
See also: