What the numbers mean

**Notional volume** — the contracts traded, counted at their $1 face value, which is how the source defines it for every venue. It is not the cash that changed hands. **Open interest** — the value of contracts still live and unsettled: money currently at stake rather than money that moved. Each venue's latest daily value on or before the as-of day. **Unique users** — separate accounts active in the period, as counted by the venue. Someone active on two venues counts twice. **Transactions** — individual fills recorded in the period, whatever their size.