One of the slightly unusual features in illumos, and Solaris because that's where it came from, is isaexec.
This facility allows you to have multiple implementations of a binary, and then isaexec will select the best one (for some definition of best).
The full implementation allows you to select from a wide range of architectures. On my machine it'll allow the following list:
amd64 pentium_pro+mmx pentium_pro
pentium+mmx pentium i486 i386 i86
If you wanted, you could ship a highly tuned pentium_pro binary, and eke out a bit more performance.
The common case, though, and it's actually the only way isaexec is used in illumos, is to simply choose between a 32-bit and 64-bit binary. This goes back to when Solaris and illumos supported 32-bit and 64-bit hardware in the same system (and you could actually choose whether to boot 32-bit or 64-bit under certain circumstances). In this case, if you're running a 32-bit kernel you get a 32-bit application; if you're running 64-bit then you can get the 64-bit version of that application.
Not all applications got this treatment. Anything that needed to interface directly with the kernel did (eg the ps utility). And for others it was largely about performance or scalability. But most userland applications were 32-bit, and still are in illumos. (Solaris has migrated most to 64-bit now, we ought to do the same.)
It's been 5 years or more since illumos removed the 32-bit kernel, so the only option is to run in 64-bit mode. So now, isaexec will only ever select the 64-bit binary.
A while ago, Tribblix simply removed the remaining 32-bit binaries that isaexec would have executed on a 32-bit system. This saved a bit of space.
The upcoming m32 release goes further. In almost all cases isaexec is no longer involved, and the 64-bit binary sits directly in the PATH (eg, in /usr/bin). There's none of the wasted redirection. I have put symbolic links in, just in case somebody explicitly referenced the 64-bit path.
This is all done by manipulating packaging - Tribblix runs the IPS package repo through a transformation step to produce the SVR4 packages that the distro uses, and this is just another filter in that process.
(There are a handful of exceptions where I still have 32-bit and 64-bit. Debuggers, for example, might need to match the bitness of the application being debugged. And the way that sh/ksh/ksh93 is installed needs a slightly less trivial transformation to get it right.)