A metric layer is a semantic layer meant for individuals, small groups, and developers looking to build their own enterprise-ready tooling around metrics. A metric layer is primarily used by data teams and technical users because it includes defining the data model which the metrics are based off of and the ability to understand the resulting abstractions that are created from that data. Developers of a metric layer can use it to build their own tooling for end users should they choose.
The metric layer only encompasses the core creation of the metric and surrounding datasets, it does not encompass the broader functionality around metric workflows by business user (the full set of features is referred to as a metric store). MetricFlow is the open sourced version of Transform metric layer. It powers Transform's metric store, which includes robust functionality on top of the metric layer, like Transform's user interface and first-class integrations.