If you want to store immutable data, which HubSpot data representation tool is the best?

In the HubSpot Data Integration certification, timeline events are the correct answer because they are purpose-built to record interactions that are "immutable," meaning once the data is logged to a record's activity feed, it cannot be changed or modified. Unlike custom properties, which are designed to store only the "current state" of a record and are overwritten every time new information is received, timeline events preserve a permanent historical audit trail of specific actions—such as a webinar registration or a video view. This makes them the ideal representation tool for high-frequency data that must remain static to provide reliable context for list segmentation and workflow enrollment without losing historical fidelity to system updates.