Both the Item Catalog and the Resource Catalog use formulas. These may be calculated differently, using different variables, according to which catalog they appear in. For example, in the Item Catalog, an Area may be defined as perimeter multiplied by thickness in the Item Calculations pane. In the Resource Catalog, that same Area may be defined as width plus length multiplied by thickness.
Quantification uses several types of variables, which are mined from model properties (if available) and the Item and Resource catalogs. For most calculations you are likely to use the model properties which are tied to takeoff objects.
Variables in model properties
These reflect the properties that are shown in the model when an object is taken off. These are the properties brought into the model from the original design application, providing they are saved at the time. They are automatically bound to a model item when it is taken off. These variables have the suffix Model, for example, ModelLength, ModelWidth, ModelVolume.Item and Resource Variables
The variables in the Item and Resource catalogs may be:
Item variables
Linked to model variables, for example, Area = ModelArea in the model, unless the variable is overridden manually or if the formula is changed.
Resource variables
These are different from Item variables, but they may also reflect the variables in the model, for example, Area = ModelArea in the model, unless the variable is overridden manually or if the formula is changed.