According to Esri (documented in this link), the Protal for ArcGIS Server 10.3 needs only ArcGIS Server 10.3 Advanced License. I tried to install the portal on a production server with ArcGIS Server 10.3 installed and licensed (advanced license). However, after completing the installation of the portal, it asked me about the license and I used the ecp file for ArcGIS Server 10.3, it accepted it; but when opening the portal it asks me for authorization. Does this mean that there is a separate license for the portal?