Relative Drawdown is the highest Max-Drawdown in Percentage during the test period.
Example: You start with 100$. On your first trade your Equity goes down to 1$. You just suffered a 99% Maximum Drawdown and Relative Drawdown will Lock that as the highest draw-down it's ever seen in Percentage.
Then your luck turns around and price spikes into your direction bringing you to 1000$. But you say No-"I'm not gonna close that order until I make it to $10,000. But the market says, "I'm due for a retrace" and starts going against you again and Equity drops to 800$.
At this point Fear kicks in and you say "Enough-is-Enough" and close the order. The Maximum-Drawdown as defined is 200$ Dollars. Yes Maximum Drawdown looks at the Cha$-Chang$. And where was Equity before it toke that draw-down, 1000$. The Max-Drawdown in % is 20 ($1000-$-200 which brought you to $800).
Relative draw-down does not care about the 20% because thats not the Highest Draw-down it's seen in Percentage. So it reports what it'd seen which is 99%.