If the desired parameter does not appear when creating a filter rule, what should the designer check first?

When a desired parameter does not appear during the creation of a filter rule, the first aspect a designer should check is the categories that are included in the filter. In Revit, filters are applied to specific categories, such as walls, doors, or windows, and the parameters available for filtering are determined by these categories.

If the parameter belongs to a category that has not been included or selected in the filter settings, it won't show up as an option. Ensuring that the appropriate categories are selected will make the relevant parameters available for use in the filter rule.

The focus on categories is essential, as other components like the operator, AND/OR settings, or view types may not influence whether the parameters are available to select. Categories directly define the scope and available data for the filter, making this the first and most crucial element to review.