In the past, Transmit Beamforming (TxBF) was a very powerful way to deliver quality service to 802.11 clients in noisy environments. TxBF uses RF properties to "steer" RF transmission at where the transmitter believes the receiver to be.
The MCS Index is a way of comparing and ranking modulation and coding schemes within, and to some degree between, PHY(s).
MCS is an interesting thing to track because it correlates with signal quality. The higher an MCS Index that is used in a transmission, the more susceptible to interference it is going to be.
Some systems do not report MCS directly as an index however. When working with such systems, one can still compute an MCS estimate directly. Here's how.