When you subscribe to a trading system, you will see trading instructions that include these abbreviations. Unlike most brokers, Collective2 specifies whether each order is an opening or closing order. This:
BTO: "Buy To Open" - Buy in order to establish a new long position
STO: "Sell To Open" - Sell short in order to establish a new short position. (For stocks, abbreviation becomes SSHORT).
BTC: "Buy To Close" - Buy in order to cover (close) an already-open short position.
STC: "Sell To Close" - Sell in order to exit and already-open long position.