1. Unilateral means only buying or selling at the same time. That is, buy only when bullish and sell only when bearish. So unilateral can be divided into single buy (long) and single sell (short).
Arbitrage refers to buying and selling at the same time. In other words, we don't predict the rise and fall of the market, while buying one variety, we sell another related variety and earn the difference between the two varieties.