There are three ways you can add formulas to line items in a model.
Individual line item formulas
Individual line item formulas apply to specific line items. You can select to include all or only some of the model versions in the formula scope.
Formulas that apply to a version in a module
Each line item you want to apply this type of formula to must have versions applied and belong to a line item subset.
Version dimension formulas
Each line item you want this type of formula to apply must have versions applied to it. This only affects line items with a numeric data type.
These also apply to a specific version but their scope of application is the entire model.