The general call addresses all devices on the bus using the I2C address 0.
If a device does not need the information provided, it simply does nothing. Devices processing the message acknowledge this address and behave as slave receiver.
The master cannot detect how many devices are responding to the message.
The second byte contains a command. The possible commands are described in the I2C specification chapter 3.13. The value 0x00, for example, triggers a software reset.