“MCC” stands for Motor Control Center. An MCC houses multiple motor control and feeder units in a modular arrangement.

A feeder bucket is a section (or “bucket”) in the MCC that feeds power (rather than directly controlling a motor). It contains the disconnecting device / circuit breaker, bus connections, etc.

This particular feeder bucket is from Square D’s Model 5 line.

The “12-inch” refers to the vertical module height (i.e. the “bucket height” or “unit size”) used in the MCC line.