Unity build vs DirectToRift build?

I'm sorry to ask the stupid question but I cannot find any document related to this. When I build in Unity using Oculus Utilities package, there are 2 file exe, one is <App_name>.exe, one is <App_name>_DirectToRift.exe. What is the difference between 2 files?
On the other hand, in Player Settings, I don't enable the option Virtual Reality Support, how can Unity know that I using Oculus Rift to build to generate the <App_name>_DirectToRift.exe?

 

The DirectToRift executable is generated when using the old Oculus Plugin for Unity (needed for Unity versions before 5.1). Search for the OVRShimLoader script if you need to change anything to how it's generated. It's used to launch the game directly on the Oculus, without having to manage the extend display mode.

If you have the Virtual Reality Support option, then you're certainly running a Unity version that don't need anymore the Oculus Plugin. When ticking Virtual Reality Support, your main camera will render two images that will be sent to the oculus when Unity finds one during the loading of your game.

 

How about unity 5.1 version? i checked the virtual reality supported box but when building project there is only one exe and there is no DIrectToRift exe. The built project cannot be used to the Oculus...