13 refers to the stock index futures contract delivered in March 21.
912 refers to the stock index futures contract delivered in December 29.
the main contract now is 912. Generally, the main contract is the futures contract of the month with the largest trading volume among the related futures contracts. Generally, stock index futures contracts will only be traded at the end of March, June, September and December six months in advance, while other months will only be traded at most two months in advance.