Effectively and practically, nothing. Specifically though there are differences;
- In the xSIID, the NTAG I2C chip that was chosen has 2k of user programmable memory space vs the 1k in the NTAG216, but only 1k is usable for NFC tap applications in the xSIID for reasons.
- The NTAG I2C family of chips do two things the NTAG2xx family of chips do not;
- Basic unconfigurable field energy harvesting and sends it to an output pin
- Can communicate with an I2C controller (host) as an I2C target (peripheral)
In short, we use the power output pin of the NTAGI2C to power the LED in the xSIID. The I2C capability of the NTAGI2C is somewhat useless for most implant applications because it was designed to be put into things like bluetooth speakers or other things that have a controller that might want to talk to phones over NFC for automatic pairing (for example), so the chip cannot act as a controller itself to read things like sensors etc.
Other than that, the configuration options, password features, etc. of the first 1k of the NTAG I2C chip and the NTAG216 are identical. If you want to access memory above 1k in the NTAG I2C chip in the xSIID, you can do it but you need to use raw NFC commands to do it. That’s totally possible to do, so anyone could write an app or use RawNFC to access and read / write those memory blocks, but it’s just not natively readable with current phone implementations of Type 2 NFC tags.