A control frame soliciting a simultaneous uplink transmission from one or more stations.
Reading a capture
Worth composing on a non-HT PPDU rather than an HE one, which looks wrong and is not: the frame has to be readable by every station on the channel, including ones that have no idea what 802.11ax is, because they all need to defer for the transmission it is about to cause.
Subfields
Common Info — Everything that applies to all of the triggered stations at once: what kind of response is wanted, how long it must be, and how it must be modulated.
User Info — One station's instructions: who it is, which subcarriers it gets, and at what rate and power it must transmit.
Further User Info fields — One more per station being triggered, each structured exactly like the one above.
Padding — Optional padding, so a station has time to read its own instructions before it must start transmitting.