Behavioral vs declared customer data
Declared data is what a customer states about themselves, like preferences set at signup. Behavioral data is what they actually do, captured as events. Declared data ages; behavioral data keeps a profile current.
Declared data
Declared data is what a customer tells you directly: the fields they filled in, the preferences they set, the plan they chose. It is accurate when given and easy to store, but it ages. People do not come back to update a form when their situation changes.
Behavioral data
Behavioral data is what a customer does, captured as events: pages viewed, items added, searches run, features used. Nobody has to report it, and it reflects the present. The cost is that it arrives constantly and has to be collected and computed into something a profile can hold.
Why a profile needs both
Declared data says who someone is and what they said they wanted. Behavioral data says what they are doing now. A profile on declared data alone describes a customer as they were at signup. Behavior is what keeps it describing the customer in front of you.
See it on your own traffic
Add the SDK, define one attribute, and see an attribute updating against your own traffic in the first session. The free tier runs on the real engine: no card, no sales call.