Select the marketplace where you expect the majority of your sales to occur. 

Your book will be sold in all marketplaces, regardless of your selection. Your choice here may affect the categories available to you in the next section For example, selecting amazon.com as a Primary Marketplace may give you access to ‘Football (American)’, whereas on amazon.co.uk, ‘Football’ relates to soccer, so a book about American Football needs to be categorized under ‘American Sports’.  

You can change the primary marketplace at a later date, even after publication. 

A screenshot from the KDP publishing process. The heading is 'Primary marketplace'. There is a drop-down box which, when clicked, will display the various marketplaces available.
A screenshot from the KDP publishing process. The heading is 'Primary marketplace'. The drop down box has been clicked and there are various Amazon market places displayed, starting with 'Amazon.com', 'Amazon.in', and 'Amazon.co.uk' etc. The drop-down options have been highlighted in green.