Such as soybean oil and palm oil. There can be some substitutes in use (for example, all can be burned). So there is a normal relationship between their prices.

Arbitrage means that when the prices of soybean oil and palm oil are high or low, according to the normal relationship, the difference between them will definitely be even, so you will try your best to fight for the difference between them. This is cross-species arbitrage.

Cross-month arbitrage is similar.