I recently installed Visual Studio 2015 and selected the Office developer tools feature as part of the install. Then I went about creating a sample SharePoint App project. The project creation wizard started with asking for SharePoint site and type of App
Next step required to specify the target SharePoint version. This step is newly added in Visual Studio 2015 and the target SharePoint version is auto selected based on the URL of the SharePoint site mentioned in previous step
As described in the step Visual Studio 2015 Office Developer Tools adds correct reference to version SharePoint client DLL based on target SharePoint version.
If SharePoint 2013 was selected for target version then visual studio adds reference to version 18.104.22.168 of SharePoint.Client DLL
If SharePoint online was selected for target version then visual studio adds reference to version 16.1.3912.1204 of SharePoint.Client DLL