The data rate display format employed for 802.11ac is, 

Format: aX.YcZ

X = mcs (modulation coding scheme) 0-9
Y = No of spatial streams, 1-3.
Z = Bandwidth 2, 4, 8 (2 = 20Mhz, 4=40Mhz, 8=80Mhz)
c = b for Beamform, s for STBC, - none
s = Guard Interval, indicates Short guard interval applicable over LGI (Long guard interval)

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The data rate display format employed for 802.11n is,

Format: mX-Z
X = mcs (modulation coding scheme) 0-23
Z = Bandwidth 2, 4 (2 = 20Mhz, 4=40Mhz)

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In the table HT MCS Index is for 802.11n, and VHT MCS Index is for 802.11ac