Does target display mode allow you to extend your desktop or just mirror it?
can I extend the desktop with my new 2015 retina display imac and a 2011 iMac in target display mode?
iMac with Retina 5K display, OS X Yosemite (10.10.3)
Target Display Mode- Frequently Asked Questions (FAQ)
Target Display Mode 2011 & Later iMacs
iMac- How to use your iMac as a Target Display Mode display
It can be used to mirror a display. I don't believe it extends the Desktop, although I have seen this discussed elsewhere, so I cannot say positively.