The separate modeling environment inside Revit - opened by editing a family rather than a project - where Families actually get built: reference planes and lines first, then parametric geometry, types, and formulas layered on top. It looks similar to the main project environment but behaves differently in important ways, most notably that a family file has no worksharing and is designed to be flexed and tested in isolation before being loaded into a real project.

Comfort in the Family Editor is usually the clearest dividing line between someone who "uses" Revit and someone who can genuinely solve problems in it - most everyday frustrations with a missing or wrong component get fixed here, not in the project file.