Keywords are one of the most important factors in the success of your Amazon Advertising.  Keywords are only used in Manual Campaigns and they are housed at the ad group level.  

Keywords are typically descriptive of the products within the ad groups.  They can also house terms referring to a seller's 'Brand' or their 'Competitors'.  It is essential that you transfer high performing customer search terms as keywords within refined match types to ensure you are competitive in the marketplace.

Keywords have 3 different match types: broad, phrase, exact. 

Additional Resources:
What is the difference between a search term and keyword?
Automatic Campaigns vs Manual Campaigns
Search Term vs Keyword

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