Lightning App Builder is the tool for creating Lightning pages, the customized screens users see in Lightning Experience and, for some types, the Salesforce mobile app. Each page has a type that decides what it is for: an object record, Home, an app landing page, or a specialized workspace such as email, forecasts, Omni Supervisor, or Command Center. You set the type when you create the page and cannot change it later, so choose the type that matches the surface you need to customize and where that page can run.