Apps that support login can share with us the user's ID. We internally call it User ID. More about user login via our SDKs can be found here.
Once this information propagates to our infrastructure it will be accessible via our REST API Analytics Endpoints as a dimension. This dimension is user:id.
An example (users that received behavioral tags during the last hour):
This can help you enrich your CRM data with User behavior, or it can help you build new visualizations in your BI tools. Check all dimensions per domain to see what can help your business further!