What information replaces signal-to-noise ratio when using the FT8 or FT4 modes in a VHF contest?
Why Standard FT8/FT4 QSOs send a signal report expressed as signal-to-noise ratio in dB (for example -15). In VHF/UHF contests the required exchange is the four-character Maidenhead grid locator, so WSJT-X's contest mode substitutes the grid square in the message slot where the SNR normally goes. This works because FT8 messages are only 77 bits, so one fixed field carries whichever piece of information the contest needs.
Watch out An RST report is a human-judged readability/strength/tone value used on CW and SSB; FT8 decodes are machine-measured, so RST is never sent. Serial numbers appear in FT4/FT8 RTTY Roundup contest mode, not VHF contests.
VHF contests score on grids, so FT8 sends the grid where the dB report would go.