The client can send a license preview request, meaning that the application can carry out a preview operation before asking the user to buy the content in order to determine whether the user's machine actually meets all the criteria required for playback. License preview refers to the client's ability to preview the license (to see what rights the license allows) as opposed to previewing the content (viewing a small portion of the content before deciding to buy). Some of the parameters that are unique to each machine are: outputs available and their protection status, the runtime/DRM version available, and the DRM client security level. The license preview mode allows the runtime/DRM client to test the license server business logic and provide information back to the user so he can make an informed decision. Thus the client can see what a valid license looks like but would not actually receive the key to decrypt the content. Support for license preview is optional, and only necessary if you implement a custom client that uses this functionality.
To determine whether the client sent a preview request or license acquisition request, call LicenseRequestMessage.getRequestPhase() and compare it to LicenseRequestMessage.RequestPhase.Acquire