MAS is a format that works with Performer from MOTU.
Perhaps MOTU Audiosuite = "MAS"?
RTAS is a format known as real time audio suite and is like the poor mans TDM plug-ins that run under protools.
Protools has 2 different types of rig.
The high end kind (TDM) for the pros usually has PCI cards which you install in your mac. They have a certain number of DSP chips on them and the effects run off these rather than take up your CPU.
This allows some unique features that can't be offered to the consumer market.
RTAS works with Protools LE the lite version usually found with a Digi 001 or 002 and the other consumer products.
The effects need your CPU to run and can be pretty hungry.
A 1176 compressor on a 450mhz mac takes more than 70% cpu and when you hit stop it takes a while to respond.
When you can get a similar effect in a VST and it is still responsive, you wonder why the RTAS is such a dog to use.
I would say all the formats now work on OSX as performer and pro tools are out for osx now.
Oh and RTAS is known as real time because the original audiosuite plugs were used by taking your audio and processing them with a processing dialog up on the screen.......similar to the way you could do effects in sound edit years ago. They were offline effects.
According to the MOTU website they can support the TDM hardware....so if you like the DSP power and don't like the protools software you have a choice.
So there is audiosuite, rtas, tdm, vst and audio units.
Out of them VST is probably the most common and audio units catching up.
sorry about the ramble.....
scoot.