for SD, SPI, Wire, and others, provide 'compatibility' versions that plug into anything expecting the atmega library, that actually compile and run using the default interfaces defined in the pins_arduino.h file for the hardware.
In the 'XMega' versions, support for multiple hardware interfaces exists (such as multiple TWI and SPI hardware objects). This is not 100% compatible with the atmega libraries that might reference it. Because libraries can now be platform-specific in the Arduino IDE, new libraries with atmega-compatible names can either wrap the 'XMega' versions, or implement them in atmega-compatible ways.
for SD, SPI, Wire, and others, provide 'compatibility' versions that plug into anything expecting the atmega library, that actually compile and run using the default interfaces defined in the pins_arduino.h file for the hardware.
In the 'XMega' versions, support for multiple hardware interfaces exists (such as multiple TWI and SPI hardware objects). This is not 100% compatible with the atmega libraries that might reference it. Because libraries can now be platform-specific in the Arduino IDE, new libraries with atmega-compatible names can either wrap the 'XMega' versions, or implement them in atmega-compatible ways.