He's talking about the binary compatibility of the various IBM system lines. The binaries of AS/400 systems include an intermediate code binary representation

He's talking about the binary compatibility of the various IBM system lines. The binaries of AS/400 systems include an intermediate code binary representation of the program that can be re-translated into a binary for a newer system when one is developed. If you wrote a POSIX based program in in the past on the AS/400, it would almost certainly run fine on the newer systems. Likewise, AS/400 programs written for AS/400 or iSeries chip based systems will run on the modern Power series OS/400 systems with a recompile. There are good and bad sides of choosing to build this level of compatibility, but IBM has made doing so a mainstay of their business for a long time.