If you are referring to AVR register symbol names like PORTx PINx etc... Those are not Arduino symbols and are not part of anything Arduino.
They are part of the avr-gcc package and are processor specific, which is outside of the Arduino "language", no different than using inline assembler.
While it can be useful, it is stepping outside the Arduino environment which provides a portable abstraction layer and therefore is not portable since it will not work on other platforms.
--- bill