Hey Jules,
The man saying stable is on the money.
The other part is proper multi-tasking, where the background tasks continue working away in the background at a determinable rate, rather than os9.what.ever's way of stealing most of the clocks for what's in the focus.
This gets more and more important as the machines get faster and more powerful, and we ask 'em to do more an' more simultaneously.
Before anyone else points out I don't know too much specific to macs, I will, but everything above aplies to unix, which is what osX is based around.