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.