They tell you the formula. For a group like this it’s usually just a percentage of your profits.
I can claim with full certainty that comp is entirely discretionary and is not beholden to any "percentages". They pay you min(market worth, amount to make you stay) - and those figures are reviewed every year.
Percentages only apply to discretionary traders who are allocated a book to trade. They get to keep a certain percentage. For everyone else, especially techies and researchers, it's discretionary.
Sort of. Traders have successfully sued when screwed on a formulaic bonus. If it's discretionary, and historically didn't follow a formula, an employee has much less to stand on.