Opening / closing
Whether a trade creates a new position or offsets an existing one. The tape does not show which, so scanners infer it from open interest.
An opening trade adds a position, such as a buy to open or a sell to open, and a closing trade reduces one, such as a sell to close or a buy to close. The distinction matters because a large call sale that closes an earlier long position says something different from a new short call.
The tape reports price and size only, so scanners infer opening activity from volume against prior open interest and from the next day’s open interest change. That inference stays uncertain at the moment of the trade.
Illustrative example (hypothetical numbers, not a real trade): a contract starts the day with open interest of 500, and a single trade of 3,000 contracts prints. Because only 500 contracts were open, at least 2,500 of those contracts were opening on at least one side of the trade.