How is a custom view defined in Revit?

A custom view in Revit is defined by creating and naming a new view from the 'View' tab. This process allows users to tailor their projects to meet specific requirements by establishing a unique view perspective that can include specified visibility settings, graphic styles, and view templates.

When a new view is created, you have the flexibility to define its properties, including scale, visibility of elements, and the specific portions of the model you want to be displayed. Naming the view also plays a crucial role in organization and navigation within larger projects, allowing team members to easily identify the custom views they need.

While adjusting viewport settings, modifying existing views with filters, or adding annotations and tags are important aspects of managing views in Revit, they do not constitute the establishment of a "custom view" in the context defined by the question. A custom view specifically originates from the creation process within the 'View' tab.