My understanding is 5.56 NATO is higher pressure than .223 REM, so barrel stamped .223 “cant” tolerate the pressure of 5.56. Barrel stamped 5.56 NATO can tolerate both 5.56 NATO and .223 REM.
Very interestingly, there is a CHAMBERING called .223 Wylde, which has a better fit to cartridges and can take both 5.56 NATO and .223 REM safely and with high performance. This chambering can be found in the MRA Renegade.