When you install Portal for ArcGIS, the account you specify during installation runs the Portal for ArcGIS service and is known as the Portal for ArcGIS account.
If you want to configure the Portal for ArcGIS account to run under a different account, you can do so after the installation has completed with the configureserviceaccount utility. This utility will also apply the appropriate permissions to directories used by the service.
To learn more, see Changing the Portal for ArcGIS account.