Custom Dimensions can be used in which reports?

Custom Dimensions are flexible and can be applied in several ways within Google Analytics. They can be used as secondary dimensions in Standard Reports, allowing you to segment existing data with your custom criteria. For example, you could see page views (a standard metric) broken down by 'User Type' (a custom dimension). They can also be used as primary dimensions in Custom Reports, giving you the ability to build reports from scratch with your custom data as the main organizing factor. This allows for deep, tailored analysis. Custom Dimensions cannot be used as primary dimensions in Standard Reports because Standard Reports have predefined primary dimensions. They also cannot be used as custom metrics; Custom Dimensions categorize data, while Custom Metrics quantify data.