Which type of family is specifically tied to a project and cannot be shared across projects?

The type of family that is specifically tied to a project and cannot be shared across projects is the in-place family. In Revit, in-place families are unique to a single project and are created within that project environment. This means that they cannot be loaded or shared with other projects, as they are designed to meet specific needs that may vary widely from one project to another.

In-place families are useful for creating custom elements that don't exist in the standard Revit libraries, allowing designers to model complex or unique components that are integral to the current project's design. Because of their project-specific nature, any adjustments or modifications made to these families remain confined within that project, reinforcing their uniqueness.

In contrast, loadable families can be created and used in multiple projects, while system families are built into Revit and cannot be loaded or modified by users. Shared families can indeed be utilized across projects, allowing for greater flexibility in design work. This distinction emphasizes the role of the in-place family as a customizable solution specifically tethered to its originating project.