How Quant Arena measures it
We count the days on which the strategy's return was not zero. Days in cash have exactly zero return, so this is a close proxy for the share of time a position was open. Strategies with less than 5% exposure in a window are not ranked by the Arena Score for that window.
Why it matters
A strategy that is invested half the time can match buy-and-hold returns with half the risk, or it can miss the best days of a bull market. Exposure tells you which kind of strategy you are looking at before you compare returns.