user_data: quick per-user key/value storage. Best for lightweight, user-specific preferences or details you just want to keep track of.

user_property_data: a richer property system. Values are tied to elements and categories, making them suitable for feature flags, group-level settings, or other configurable attributes that need metadata (type, visibility rules, etc.).

In short:

Use user_data for tracking simple, per-user preferences (e.g., “prefers dark mode”).

Use user_property_data when you need configurable, metadata-driven flags that can live at different scopes (user, group, customer).

Example:

If you want to remember that a specific user likes “compact view,” store it in user_data.

If you want to mark some users as “admins” using a feature flag that can be managed in Central/Console, create a user_property_data entry like FeatureUser/Admin. This gives you a toggleable flag that can be applied at the user, group, customer, or combined scopes (via FeatureUser, FeatureGroup, FeatureGroupCust, FeatureGroupCustUser).