The xNT is just an NTAG216 chip that you can write small bits of data to (called NDEF records). It works for text, URLs, long passwords, or maybe a small image.
The xDF2 has much more memory space (about 10x more IIRC). It’s a more advanced tag that is intended to have little applications frameworks. It’s a bit more involved to set up, but once you do you can use it just like the xNT, only bigger. Here’s a thread where briefchase talks about setup (it’s for the flexDF, but they use a similar chip AFAIK):