• impression_id
• request_tag
• user_id
• purchases

The correct answer is: user_id

Explanation: Overlap analyses in AMC enable you to generate insights by comparing different groups of user_id values in isolation and when overlapped. The user_id field is the pseudonymous ad identifier that links impressions, clicks, and conversions to the same anonymous user, allowing for audience overlap measurement.

Reference content: From the official Amazon documentation (Enhanced scoring for overlapping AMC audiences):

“Audience overlap analysis is a powerful tool that helps advertisers understand how their target audiences intersect and relate to one another. … Identify the top 3 overlapping audiences with your AMC audience … using user_id values to measure overlap and calculate affinity and percentage scores between audience groups.”