Released on May 10, 1999, Mac OS 8.6 introduced the concept of a nanokernel to the Mac OS which allowed for multi-tasking and multi-processors. As such, it was the first version of the Mac OS to include the "Mac OS ROM" file in the System Folder. This file was loaded for "New World" Macs that didn't contain a ROM in hardware, and also overrode the built-in ROM when loaded on "Old World" Macs.
This free update for Mac users running 8.5 and 8.5.1 was faster and much more stable than either versions of 8.5.x, and is by some considered the most stable Classic OS.
It was also the first Mac OS to have the OS version displayed as part of the MOSS (Mac OS Startup Screen). Many hardware upgrades require a minimum of Mac OS 8.6.
Mac OS 8.6 had some incompatabilities with the version of the "Classic Environment" used in Mac OS X Server 1.0 where MacOS.app could become unstable after quitting a piece of Classic software.