At a certain point in time, several contracts of a variety of futures can be traded at the same time, ranging from 4 to 5 to 12. But not every contract is active, usually only 1-2 contracts are active, that is, the main contract and the sub-main contract. The common main contracts in oils and fats are 1, May and September contracts.
And the price of each contract is usually different, which is called short-term and long-term premium. The closer the contract, the closer to the spot price.
Didn't your book talk about the main contract?