Custom parameters added to specific categories within a single Revit project - visible in schedules and tags in that file, but not shared with other projects like Shared Parameters are. Project Parameters are the simplest of Revit's three parameter types to set up - useful for a one-off tracking field on this project only - but because they're baked into that single file, they can't be tagged, scheduled across other projects, or exported to ODBC the way a Shared Parameter can.