Expressions and aggregates are both definitions created in the Data manager, and both are evaluated from the customer’s data when used. The difference is shape, not storage: an aggregate applies one aggregation function over a customer’s events (sum, count, first, last), while an expression is a formula that can combine up to 26 attributes and aggregates with logic and arithmetic, for example turning a last-purchase timestamp into days since last purchase.

In practice

Use an aggregate to reduce event history to a number, and an expression to combine or transform values into the form a segment or campaign actually needs. When a derived value looks wrong, check its inputs first: an expression is only as good as the attributes and aggregates it is built from.