TAS - Trade at Settlement Order type is an interesting order type that was introduced for majority of the agricultural futures contracts in 2015. How is TAS different from other order types ?

CBOE gives a good primer on this order type

Trading at Settlement (TAS) is an order type that allows a market participant to buy or sell futures contracts during the trading day equal to the yet-to-be determined settlement price, or at a price up to 4 ticks above or below that price.

VIX futures also have TAS variation and have a suffix T at the end of futures identifiers such as VXT, VXT15, VXT17.