The interested reader can refer the first part of the series here — <802.11ac PLCP Frame Format – Part 1>
VHT-SIGB Field
The VHT-SIGB format provides the Length and MCS rate that the 802.11ac frame was transmitted at in a Multi-User (MU) transmission. The VHT-SIGB field is one symbol and can be 26 bits in a 20 MHz PPDU, 27 bits in a 40 MHz PPDU and 29 bits in 80/160/80+80 MHz PPDUs. The description of VHT-SIGB and the varying lengths are explained in the standard and provided for reference below
Fig Courtesy: 802.11 Standard
For a description of how the different parameters of VHT-SIGB are calculated and inserted into the VHT-SIGB field – refer section 22.3.8.3.6 VHT-SIG-B definition in 802.11ac-2013 standard.