Tick volume
Trader's Glossary · Contracts and markets

Definition. The number of price changes in a given period at a single quote provider, shown on many forex and CFD platforms in place of real volume.
How it works
The spot forex market has no single exchange, so there's no common volume. The platform therefore counts how many times the price changed and shows that as “volume”. It can be a proxy for activity, but it doesn't tell you how many contracts changed hands or who was aggressive.
Example
On CME futures, a candle's volume is the number of contracts, split into trades at the bid and at the ask. That's what delta and the footprint are built from. This information can't be reconstructed from tick volume.
What to watch out for
Volume indicators, delta or a “footprint” calculated from tick volume look similar, but they're based on completely different data.

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See also
Volume, Delta, Footprint, Futures contract.
More in Hetman Capital Club
Full article in the Club (in Polish): 🏛️ Exchange or broker: CME futures vs CFDs and forex with a market maker.
You'll find the full explanation with graphics, examples and exercises in the 📖 Order flow encyclopedia space, and its use in the live market in the 🔴 Live Room. Both are available to Club members (in Polish).
Educational content, not investment advice.