A simultaneous session is more than one but limited to three, sessions of ArcGIS Pro run simultaneously by a named user. ArcGIS Pro named users are allowed up to three simultaneous sessions of ArcGIS Pro at anytime, irrespective of running the software online or offline. Portal for ArcGIS manages the number of simultaneous sessions a user can have.
Simultaneous sessions differ from concurrent use in that only specified named users can run ArcGIS Pro software. In the concurrent use environment, any installation of Desktop, Engine, Esri CityEngine, or ArcGIS Pro can access a concurrent use license if the License Manager is specified.