Who counts as a competitor
Every product on the first page of your keywords, grouped into families. Nothing is pre-selected: the whole field is in the data, with your own products as the point of view.
You don't have to pick competitors: everyone who appears on the first page of a market's keywords is part of the market. To follow particular rivals more closely, mark them as key competitors.
The whole field
Each capture keeps every organic listing on the first page. So the market contains the leader on your main keyword as well as the product that shows up on a single long-tail term, and the product climbing from the bottom of the page, which is often the one worth knowing about. By the time a rival reaches the top, it is no longer news.
Variants are grouped, so a rival selling the same product in nine colours counts once. See Variants and product families.
Your own products
The products you marked as yours are the point of view, not competitors: the movement view, for example, shows who passed you and whom you passed. If you sell several different products on these keywords, mark each of them; otherwise the ones you did not mark count as rivals.
Seeing a rival you care about
The field follows the keywords. If a competitor you watch does not appear, they are not on page 1 for the terms in your market. Add the keywords they win on and they come into view. The ASIN search in the Explorer lists those keywords for any product; see Searching the Explorer.