• conversions
• purchases
• request_tag
• engagement_scope

The correct answer is: request_tag

Explanation: The request_tag field functions as a grouping indicator for the impression and any associated clicks, view events, or conversion events. It allows you to connect related ad events across different tables using a shared value.

Reference content: From the official Amazon documentation (DSP impressions table):

“request_tag: ID that connects related impression, view, click, and conversion events. For example, if an impression served has a request_tag value of ‘X’, related conversion events will have an impression_id value of ‘X’. … It is a best practice to consider using UNION ALL instead of joining based on the request_tag to combine data sources. For insights that involve user-level ad exposure across tables, use user_id instead.”