I have installed SharePoint 2013, which should correspond, if I'm not mistaken, to the build 15.
In the GAC, some SharePoint libraries such as
Microsoft.SharePoint.dll are version 15, but some others, such as
Microsoft.SharePoint.Client.dll, are available in two forms: version 15 and version 16.
What are those libraries corresponding to the future build of SharePoint? Does it correspond to a service pack?