Each System 6 version, complete with the mathematically questionable double decimal point, is similar to what we get today when we update our systems. Every upgrade adds support for something new, fixes a bug, or a combination of the two.
This is going completely off memory, but...
6.0.0: First release of System 6. Known to be buggy. Supports 512Ke, Plus, SE, II, IIx.
6.0.1: Apparent bug fix, pulled early, very rare in the wild.
6.0.2: Stable release of System 6. Earliest version included on System 7 CD-ROM.
6.0.3: Adds support for SE/30 and IIcx.
6.0.4: Adds support for IIci and Portable. Portable cdev and Battery DA introduced. First version to park drive heads at shutdown.
6.0.5: Adds support for IIfx. 32-bit QuickDraw INIT introduced.
6.0.6: Adds support for LC and IIsi. Officially adds Classic support.* Unofficially adds LCII support. Introduces on-board sound input support in Sound cdev and includes Brightness cdev for the first time. Changes icon at shutdown screen to use information icon rather than a compact Mac (presumably for the LC, the first Mac that wasn't a compact with a rocker switch for shutdown). Has an apparent AppleTalk bug. Packed with very early Mac Classics.
6.0.7: Bug fix for 6.0.6. First version compatible with TrueType INIT. Final version included on System 7 CD-ROM (which skips 6.0.6). Unofficially adds support for PowerBook 100. First to support StyleWriter printers. Likely first version to be compatible with IIe card.
6.0.8: Adds new versions of printer drivers, lining up with System 7. Officially listed as having LCII support. Final version of SSW to run on 512Ke.
6.0.8L: International version of System 6. Runs only on certain machines, including LC, LCII, Classic, Classic II, and PowerBook 100. Officially adds PowerBook 100 support and is only version of System 6 capable of running on a Classic II (or Performa 200).
Note that there are some compatibilities not listed. Any version of System 6 will run on a Classic, although it won't include a way to adjust brightness unless you either have a newer version. (A Classic will also run versions earlier than 6). The Classic ROM disk has 6.0.3 and a prototype version of the Brightness cdev with a vertical slider (the final release has a horizontal one) and a different icon. An LCII (or any of its Performa derivatives) can run 6.0.6 or 6.0.7. (In fact, all the LCIIs in my lab run 6.0.7). I'm not sure offhand if the IIcx can go earlier than 6.0.3 or the PB100 can run anything earlier than 6.0.7. Also, the 512Ke is not officially supported by any version of SSW 6 but will run it just fine. I've run as recent of a version as 6.0.8 on mine.
I also haven't tested whether StyleWriters, TrueType, or the IIe card are OK on 6.0.6 (if anyone wants me to do that, I have a IIe-card equipped LC and a StyleWriter I here along with my copy of 6.0.6), but some programs, namely Davidson's Kid Works 2, will reject 6.0.6 and demand 6.0.7 despite the only real difference being a bug fix.
Another note on 6.0.6: it was indeed packed in with early Classics. I know it had a creation date of 9/4/90 (source: I have both System Startup and System Additions disks for it). I have a Classic with an August 1990 date and know someone who had a July 1990 Classic that also came with them. 6.0.7 has a creation date of 10/13/90, just two days before the Classic's release, and was included last-minute with the Classic. If you've ever seen a System 6 disk with a circular green sticker that says "6.0.7" on it, that disk was packed in with an early Classic along with a note to destroy your 6.0.6 disks and use 6.0.7. It was too late to take the old ones out of the box, apparently. The 6.0.6 complete disks do exist in the wild...
6.0.6 does run fine on other Macs. I tried it on both a Plus and an LC. Of course, I didn't use any networking features...didn't want to push it...
Regarding that System 7 CD-ROM: It had 6.0.2, 6.0.3, 6.0.4, 6.0.5, and 6.0.7 on it.
Apple's website hosted 6.0.3, 6.0.5, and 6.0.8.
Why go with these older versions when 6.0.8 is available?
- Unlikely, but there may be an issue with certain programs and newer SSW
- Ditto for hardware
- A program with a boot disk may use a particular version
- If you're a developer, you may want to test for compatibility with them
Nothing seems to really change in terms of RAM or storage requirements between them unless you count newer INITs, cdevs, and add-ons.
One final note: 6.0.6 was the first version on two 1.4MB floppies.