What's a "relative pair trade" for currencies?
It means you can make money trading a pair of currencies if both lose value compared to a standard. Likewise you can make money trading a pair of currencies when both increase in value compared to a standard. You mostly care about the change of a currency's value relative to another.
If you think USD will decrease in value 10% versus gold and you think EUR will decrease 15% versus gold, you sell EUR to buy USD and wait to see if you were right. You don't have to compare currencies to gold. There are other standards for what a currency is worth. It doesn't matter what the standard is because your profit depends upon the value of a currency compared to another.