Introduced following the System/34 and prior to the System/36, it was also a descendant of the fabled IBM 'FS' project, which was designed to be a followup/replacement for the System/360 and System/370.
So the System/38 was nearly called the System/380, and the AS/400 was nearly called the System/40.
System/38 and its descendants are unique in being the only existing commercial capability architecture computers. The earlier Plessey 250 was the only other computer with capability architecture ever sold commercially. Additionally, the System/38 and its descendants are the only commercial computers ever to use a machine interface architecture to isolate the application software and most of the operating system from hardware dependencies, including such details as address size and register size.