It's the Pioneer version of Audyssey, Trinnov etc... In short it's the automated calibration procedure of the receiver. I have the Pioneer SC-05 with MCACC. I like what it does (especially standing wave correction) but I prefer to defeat the EQ settings it comes up with. That's just my preference, there is no right or wrong to it. Overall I personally like MCACC - though I will readily admit I have not experienced Audyssey or any of the others available on differing receiver brands.

Dan
On-Wall M5HP LCR, QS8 & EP500 in 7.1