BTC is a M5 Base that allows you stand your M5 Core instead of laying them down or hang on the wall. BTC is not just a seat for holding the M5 Core but also comes with serveral features like temperature and humiluty detection (by SHT30), and charging base.
Although M5Stack BASIC and GRAY Core can be attached to the base, but BTC can not charge them. After plugging-in a USB cable, the Core is powering without any charger.
Once M5Stack Core has been attached to BTC, it can not connect to an ENV Unit. This is to avoid I2C address conflict with built-in SHT30 sensor of BTC Base.