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.
Reading a capture
Sixty-four bits describing a transmission that has not happened yet. The stations addressed below do not choose their rate, bandwidth, guard interval or duration — this field does, which is the inversion that makes simultaneous uplink possible at all.
Subfields
Trigger Type — What the triggered stations are being asked for.
UL Length — How long the solicited transmission must be, expressed as the LENGTH field the responding PPDU will carry.