A metric represents an aggregation. It's an expression calculated on the data of the domain. Metrics can be build using the OB1 Modeler expression, but it should always be aggregated using an aggregate function.
A metric definition can also include a condition in form of a CASE operator: for example to compute a metric on a subset of the Domain.
Grouping several metrics in the same Domain allows for better performance when running analytics.
The data scientists lab: