In IBM, S/32, S/34, S/36, S/38 were used for business purpose. Actually AS/400 was designed to replace the System/36 (S/36) and System/38 (S/38) machines, IBM's previous midrange computers.
Have a look at the following, you will get some clear picture about history of AS/400
1969 - System 3
1975 - System 32 - It has display screen and keybaord
1977 - System 34 - It supports multiple workstations, 8 local users, 256 kb memory, 13 MB disk storage
1978 - System 38 - It has 32 MB memory, 14GB disk storage, used in application development
1983 - System 36 - It has 7 MB memory, 14GB disk storage, supports 72 users
1988 - AS/400 - It is the expansion of S/38 architecture but also provided application program compatibility with S/36. System 38 technology & System 36 user interface.
The story of how the AS400 got its name is given in a book authored by John Sears.
It was originally going to be called the Application System/40, or
AS/40. IBM's PC development team complained about the name. They had
just developed the PS/2, and if the midrange group was to take the
double digit numbers they would be restricted to just ten models, PS/0 -
PS/9. Therefore, IBM's management decided that the PCs could have the
range of numbers 0-99, midranges 100-999, and mainframes would have the
range 1000-9999. Thus the AS/40 became the AS/400.