The figure is firm-specific and recalculated periodically, which is what makes it interesting: it is one of the few standardised, audited numbers a retail trader can compare across brokers. Published figures have commonly sat somewhere in the region of two thirds to four fifths of accounts.
It measures accounts, not traders and not money. An account that deposited once, lost a small amount and stopped counts the same as one that lost a fortune, and a trader with accounts at three firms appears three times.
Read alongside b-book economics it explains part of the industry's structure: where client losses are the firm's revenue, the disclosure is effectively a summary of where that revenue comes from.
Example: a broker disclosing 76% is saying that for every 100 retail accounts active over the year, roughly 76 ended the period down. That is the base rate against which a new account starts.
Related: b-book, esma-leverage-caps, fca, dealing-desk