When you're adding audience targeting layers in Amazon Ads, especially with retargeting audiences, the platform's goal is generally to deduplicate users while combining the criteria.
Think of it this way:
Combination (AND Logic): When you layer multiple audience segments (e.g., "people who viewed my product" AND "people in the 'Electronics Enthusiasts' lifestyle audience"), Amazon's system looks for users who fit all of those criteria. It's not just adding the total size of each audience together, but finding the intersection.
Deduplication: Once that combined (intersecting) audience is identified, Amazon's ad delivery system aims to deduplicate users within that final targetable segment. This means if a user qualifies for your ad through multiple paths or targeting layers within the same ad group or campaign, they won't be counted multiple times for reach, and the system will try to avoid showing them the exact same ad excessively.
Amazon provides tools like Audience Overlap Reports (especially in Amazon DSP) and features within Amazon Marketing Cloud (AMC) precisely to help advertisers understand these intersections and deduplicated reach. Their "Ads Planner" tool also offers "deduplicated reach estimates."
So, in short, they are combined through your layering logic, and then deduplicated to give you a more accurate picture of unique reach and to optimize ad delivery. This helps you avoid wasting impressions on the same person and understand the true size of your target segment.