Remember Arc/Info and coverages? Coverages were just features stored as files in a directory. Then along came ArcView with data stored in shapefiles, which was different. Then later ArcMap with the ability to store GIS data in Microsoft Access or an Enterprise Database like Oracle or SQL Server. Then came the File based Geodatabase, which was … dejavu … features stored as files in a directory.
Remember ArcView 1, 2, and 3.x? You saved your data view and multiple layouts to a project file (.APR file). Then later with ArcMap you saved your data view and one layout to an .MXD file. Then came ArcGIS Pro with … you guessed it … a .APRX project file to store all your work.
What’s next? When ArcGIS Pro first was released, you had to have a named user account in ArcGIS Online tied to an ArcPro license to use it. At version 1.2, ArcGIS Pro licensing now has more options. ESRI added licenseing the “old way”, with concurrent use and single use licensing options. For concurrent use licensing, you will need the 10.4 version of the License Manager running on a server like you do right now for your concurrent use ArcGIS Desktop licenses.