Also was the foundation of Blue Box if I remember right. (And QuickTime’s portability layer was for Carbon.)

(author) MAE isn't the basis for Blue Box, though I'm quite sure it informed its design. Blue Box/Classic is actually more like MAS, the aborted Mac compatibility layer for PowerOpen/AIX/"A/UX 4," in that it runs PowerPC code directly on the CPU in the "problem state" and uses a paravirtualized operating system and enabler. There is no processor emulation in Classic except for supervisor and faulting instructions.

There are also differences at the level they execute: MAE can, and was designed to run, as an independent process like any other well-behaved X11 application, and multiple users can run multiple sessions of it, but Classic/Blue Box needs operating system support and only one instance of it can be running on a system by a single user.