Under the current paradigm, the dollar (or any currency) serves as an intermediary to determine the relative price of goods. It's like asking how much gold you could get for a barrel of oil.
Say you want to exchange 1 barrel of oil for a certain amount of gold. You know how much gold you can get because oil is valued at a certain amount of dollars per barrel, and gold is valued at a certain amount of dollars per ounce. The dollar doesn't have any inherent value (it used to until we went to a fiat money system) - it just serves as a common denomination.
This might be an oversimplification, but this is how I think about it: Think of BTS-X as a currency like the dollar. Now think of a dollar as a commodity (just like Oil)
1 BitUSD will be equivalent to a certain amount of BTS-X
1 BitOil will be equivalent to a certain amount of BTS-X
It won't be set in stone to begin with, but by the action of many parties trading both BitUSD to BTS-X and BitOil to BTS-X, there will be an equilbrium reached between BitUSD and BitOil via BTS-X.
If there's a difference between the (PRICE OF OIL) and the (BitUSD to BitOil ratio), that would present an arbitrage gap which traders could take advantage of to make a profit. Eventually, the gap would close as more trades were made; hence, the price would tend towards the real-world equilibrium.
Edit: This was in response to both delulo and unlimited_power. I just realized that what they were asking was regarding what the unit of BitOil was, which I'm guessing is in barrels.