In Custom Reports, metrics and dimensions share Same scope in order to report accurately.
- Same index
- Same Custom Report
- Same view
- Same scope
The correct answer is: The same scope
Explanation: 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.
Read more here: https://support.google.com/analytics/answer/1033861