Event Property
Beginner PronunciationEN/ɪˈvɛnt ˈprɑːpərti/
An event property is an attribute sent with an event that says what that particular occurrence involved: which product, what amount, which page.
Properties travel with the event as key-value pairs, and each has a data type that decides whether it can be summed or only grouped by. The important difference from a user property is that an event property records the state at that moment and never changes afterwards, while a user property reflects the person's current value. Two common traps: a very high-cardinality value such as an order id or a full URL with query parameters makes breakdowns useless and storage heavy, and a property that was not sent from day one cannot be added to past events.
FAQ
An event property is attached to one occurrence and freezes that moment permanently; a user property sits on the person's profile and each update overwrites the previous value. To answer which plan someone was on at the moment of purchase you need an event property, not a user property.
Sources:Mixpanel — Events and properties