Q:

What is version 16 in the GAC?

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?

A:

what i am thinking, if you are doing some development with SharePoint Online / Office 365, then you can see the version 16 for client dll. Office 365 always ahead of race, currently if you see the version for office 365 it is 16. 
So that make sense, if their some development or development tool installed for office 365.
check this one http://blog.aptillon.com/2013/08/07/sharepoint-2013-version-16-0-0-1810/

