hi there is even v6r1 now.. some hardware have been improved and there is a difference
<i>is it the new improved hardware or the operating system changes?</i>
In detail, only IBM can say for certain. However, the basic idea is that V=Version, R=Release and M=Modification level. A new version generally has internal structural elements that have been reworked to lay foundation for new functions and even new kinds of functions. A new release has new functions that became feasible or possible in the latest version. A modification level generally doesn’t introduce new operating system function, but often enables a version or release to run on new hardware. A modification level might also have areas that have been reworked for reliability or performance, I suppose, though I can’t recall specific examples.
Any of those can have exceptions. Version 3 had V3R0M5, V3R1M0 and V3R2M0, all intended for CISC processors. In addition V3R6M0 and V3R7M0 were somewhat concurrent with V3R1 and V3R2 but for RISC processors. (There might have been one or more “M” levels that I’m skipping.)
Certainly, there are relationships between OS VRM levels and hardware. Advances are happening in software and hardware all the time. It’s sensible that there are linkages between them. Some hardware advances are what make new operating system functions feasible in the first place. Research and subsequent advances are always made with future function in mind. IBM planning should take expected release dates of both into consideration.
So, I would expect a logical connection in numbering regardless of any intended linkage.
But AFAIK, the VRM numbering primarily indicates operating system (software) function.