In Custom Reports, what must metrics and dimensions share in order to report accurately?

Not every metric can be combined with every dimension. Each dimension and metric has a scope: user-level, session-level, or hit-level. In most cases, it only makes sense to combine dimensions and metrics that share the same scope. For example, Sessions is a session-based metric so it can only be used with session-level dimensions like Source or City. It would not be logical to combine Sessions with a hit-level dimension like Page. Tip: For custom reports you must specify at least one dimension and one metric. A valid combination.