The second sector (Sector 1) of the NTAG I2C plus chip (found in the Dangerous Things xSIID implant) is inaccessible to most NFC applications. However, it's readable and writable with the NFC Shell app.
xSIID Tools provides tools to help read data from and write data to NTAG I2C chips. It can convert between ASCII text and raw ISO1443A commands for writing data, or convert hex output of a read back into ASCII.
These tools could brick your implant!
Interacting with a chip directly using raw ISO1443A commands via NFC Shell is more dangerous than using a consumer NFC writer app.
Only for use with NTAG I2C plus!
Unless you really know what you're doing, these tools shouldn't be used on any other chip but the NTAG I2C plus with 2k of memory, which can be found in the Dangerous Things xSIID implant and their NITC (NTAG I2C Test Card).