people.set()

people.set() is a method, in the style of some tools, that sets a property on the user's profile or overwrites its prior value — such as setting the user's city or plan.

people.set() is from the profile-operations family, and its fundamental difference from an event property is time: an event property locks the state at the moment it happened and never changes, while a user property set with set() always reflects the person's current value and is overwritten on each set. That overwriting creates a historical trap: if the user's plan changes from free to paid and set() overwrites it, you can no longer tell which plan they had at the time of an old purchase, unless property history is kept separately. So choosing between setting on the profile and sending as an event property is a modeling decision: put a stable current attribute on the user, but send the attribute that matters 'at that moment' on the event.