Which two categories can a project parameter be applied to?

A project parameter in Revit can be applied to loadable families as well as system families, making it critical for managing and organizing information across different types of elements in a project.

Loadable families are those that can be created and customized independently of a specific project, such as doors, windows, and furniture. Project parameters assigned to these families allow you to input and manage additional data specific to those elements, enhancing their flexibility and connection to overall project documentation and scheduling.

System families, on the other hand, are built into the Revit software and include items like walls, roofs, and floors, which are essential components of the building model. Project parameters can also be added to these families to tailor their attributes according to the project's needs, again facilitating better data management within the Revit environment.

Other options, such as units and project location, serve different functions and are not categories to which project parameters can be applied. Units relate to measurements used in the project, while project location pertains to the geographic context of the project. Thus, they do not encompass the concept of project parameters as it pertains to element categorization.