If you buy the "full" version of FrameMaker 2015, you can switch between FrameMaker and FrameMaker XML Author.
You can do this in the Preferences (> Edit > Preferences > Product Interface):
After a restart of FrameMaker, FrameMaker will be in the "FrameMaker XML Author Mode". Of course, you can also switch back to "full" in the same way.
Please note, that switching is only possible with a "full" license.