Why bettors count in units
«+400 this month» says nothing without knowing the stakes; «+4 units on 60 bets» is a complete statement. Units also encode confidence: many bettors stake 0.5u on speculative plays and 2u on their strongest, keeping the scale honest.
The one rule
The unit is defined as a share of the bankroll and gets recalculated as the bankroll moves — not resized after a bad night. A unit that grows when you chase losses is not a unit; it is bankroll management failing.