Revit's built-in tool for exploring several design alternatives - different lobby layouts, different facade treatments - inside a single project file, without duplicating the whole model. Elements get assigned to an Option Set (a group of mutually exclusive choices) and a specific Option within it; the rest of the model, outside any option set, stays the shared "main model" that every option shares.

This is genuinely different from comparing full design alternatives in generative or computational workflows - Design Options is specifically Revit's own, file-internal mechanism, mainly used for short-lived, small-scale comparisons during design development rather than large parallel-scheme studies.