This form is called from several places within Impact where you need to build an expression or formula involving Standard Variables. For example, on the Geometry Construction form you can use it to enter a formula that, when evaluated, will determine whether or not the construction will be created. Also, on the Standards Designer form you can use it to build the formula that will be used to calculate the value of a variable.
The form is normally accessed by pressing the formula button 
when it appears next to an entry field.
Expression Builder
Expression
This is the area where you build up the expression. You can do this either by typing directly into the entry field or by selecting items from the other areas of the form.
Check and Evaluate
Use this button to check the format of the entered expression and display its current value.
Functions
This area contains all available arithmetic, logical, trigonometric etc. functions. You can display a sub-set of functions in the right-hand column by selecting one of the categories in the left-hand column. The box underneath contains a description of the currently selected function, and the box below that shows the text that will be added to the Expression area when you press Insert.
Variables
This area will contain a list of all variables in the current standard. Select the required one and press Insert to add this to the Expression area.
Expression Units and Decimal Separator
This area of the form displays the units and decimal separator in use. If these are different to the user's current settings they are displayed in red.
See also the following options on the Impact Explorer > Variables Window: Use Imperial Expressions and Use Commas for Decimals.
Ok
Use this button to close the form and return the expression.
Cancel
Use this button to close the form and cancel any changes.
Clear
Use this button to close the form and clear any value that may have been present in the field before the Expression Builder was loaded.