Google Analytics Metrics

Unique Dimension Combinations is a Metric in Google Analytics under the Session section

Unique Dimension Combinations Definition: Unique Dimension Combinations counts the number of unique dimension-value combinations for each dimension in a report. This lets you build combined (concatenated) dimensions post-processing, which allows for more flexible reporting without having to update your tracking implementation or use additional custom-dimension slots. For more information, see https://support.google.com/analytics/answer/7084499.