What VHT40 means
VHT is the name of the 802.11ac physical layer, and the number after it is the channel width: VHT20, VHT40 and VHT80 mean 20, 40 and 80 MHz. VHT40 is 802.11ac modulation on a 40 MHz channel in the 5 GHz band. When a datasheet lists the link rate as VHT40, it names this operating class, not a fixed Mbps figure.
40 MHz versus 80 MHz
A wider channel gives more theoretical throughput but takes more spectrum. A 40 MHz channel uses half the spectrum of an 80 MHz one, so the 5 GHz band holds more non-overlapping channels and several systems can sit side by side. In offices and trade-show halls full of Wi-Fi, finding a clean channel often matters more for a stable picture than the theoretical peak.
The actual rate follows the channel
Two VHT40 links can run at different speeds depending on channel quality. Modulation shifts automatically between BPSK and 256-QAM: higher orders at short range with a clear path, lower orders at long range or under interference to keep the link up. The video bitrate adapts with it, so a brief drop in signal does not cut the picture outright.
ATE9430T, ATE9430R, ATE9460 and ATE9461 all list VHT40 (802.11ac, 40 MHz) as the link rate. They operate at 5.15 – 5.85 GHz with channels limited by sales region, and scan for a channel at power-up. The link rate of the 8K30 ATE9860 / ATE9861 is TBD.