This RFID tag is really unique: it works with mobile phones just like other RFID tags, but you can reprogram it over I2C. The tag shows up as an ISO/IEC 15693 (13.56MHz) chip which is readable by phones and tablets. This could be interesting in situations where you want a tag that can be re-written dynamically when connected to a controller. For example, Adafruit did a test where they had a microcontroller write different URLs a few seconds apart, and the mobile phone detected the different URLs one after the other.
Note that the most popular hobby RFID reader/writer chips like the PN532 do not support ISO15693 so you cannot use those to read the ST25DV tag once programmed! You can use both Apple and Android phones to read the tags.
According to the datasheet, one can even use it as an 'I2C to RFID' transfer system to wirelessly send and receive data from mobile devices (you'll need to write a custom app for that kind of project, though).
To make connections easy, the breakout contains an ST25DV04 chip, support circuitry and even a PCB trace antenna. There's standard 2.54mm pitch headers for use with a breadboard. Should you wish to avoid soldering, the breakout also includes Adafruit's Stemma QT connectors (SparkFun Qwiic compatible). Using these handy connectors you can simply plug in the sensor and get rolling with your project. QT Cable is not included, but we have a variety in the shop.
Technical Details
I2C interface
Two-wire I2C serial interface supports 1MHz protocol