Supply in Profit, as the name suggests, represents the absolute amount of coins in a given network which are currently in profit (i.e. the price at the time they were last moved is lower than the current price).
This metric is helpful in highlighting market tops by suggesting when investors might be close to taking profits.
It is important to note that unlike Unrealized Profit, Supply in Profit does not account for the amount of profit - only the binary case of whether or not the coin is in profit.
Supply in Profit is calculated by determining which coins were last moved when the price was lower than the current price (i.e. the price has increased since the coins were last moved). It represents the total absolute number of those coins.
Rafael Schultze-Kraft - Dissecting Bitcoin's Unrealised On-Chain Profit/Loss