Ceiling-hosted families in Revit are built using templates that contain a host element, which is represented by a reference plane. When an instance of a ceiling-hosted light fixture is placed in a project, it must be associated with, or hosted by, a ceiling element. The family's internal geometry is constrained relative to the reference plane that represents the face of the ceiling host. This hosting relationship is the fundamental condition that allows the fixture to be placed and to move with the ceiling if the ceiling's position or elevation changes.

A. Snapping to nodes or grid lines on a ceiling is a placement aid for alignment but is not the fundamental requirement for hosting.

C. A light fixture can be placed anywhere on a ceiling surface, not just on points defined by the ceiling's grid pattern.

D. In a typical multi-disciplinary workflow, the ceiling exists in a linked architectural model, while the light fixture is placed in the host electrical model.

1. Autodesk, Inc. (2023). Revit 2024 Help. "About Placing Elements on a Host Component". This document explains that host-based elements, like ceiling-hosted fixtures, require a host element (a ceiling) for placement.

2. Autodesk, Inc. (2023). Revit 2024 Help. "Create a Loadable Family". The documentation notes that family templates (e.g., Ceiling-based.rft) include a host element represented by reference planes to which the family geometry is constrained.