Two OFDM symbols carrying a version number and the parameters every generation needs, in a layout later amendments must preserve.
Reading a capture
The structural idea of 802.11be. Its first field is a version identifier, and the fields around it are split into a version independent part that every future amendment must keep, and a version dependent part each may redefine. That is the first time the PHY preamble has had an extension mechanism rather than a convention.
Subfields
PHY Version Identifier — Which generation this transmission belongs to. Zero for 802.11be.
BSS Color — A short identifier for the transmitting BSS.
TXOP — How long the transmit opportunity lasts, readable without decoding any MAC header.
Version dependent fields — The half of U-SIG that each amendment defines for itself — for 802.11be, the PPDU type, the EHT-SIG size and coding, and a CRC.