Revit's system for modeling a building across time rather than as one static snapshot. Every element gets assigned a Phase Created and, if relevant, a Phase Demolished, and every view is set to show a specific phase (or a phase comparison, like "New Construction" showing new work highlighted against a greyed-out existing building). This is essential for renovation and retrofit projects: the same model can represent existing conditions, demolition, and new work simultaneously, with each view filtering to show only what is relevant.

Phase Filters control exactly how each phase state gets drawn - what line style demolished elements get, whether existing elements show at all - giving full control over how the story of the building's timeline gets told on paper.