To ensure legitimate sellers for some sales categories, Amazon will prevent a seller from listing the product until they submit their application and get approved by Amazon. These category applications will likely ask for distributor/manufacturer invoices, business license and other documents showing/proving your a true business reseller. These gated categories are completely locked for all products in them, until Amazon approves your seller status in the category.
Ungated categories are those that do not have the above restriction; allowing sellers to list without an approval process. However, there are also something called ‘Brand Restrictions’ which could exist in ungated categories. For example, ‘Electronics’ is an ungated category, but you will find restrictions on listing Apple, Bose, and other brands when attempting to list. Sometimes these restrictions are for ‘new’ conditions only while others are for the brand in any condition. When attempting to list, the restriction will be shown along with a button to click and submit an application.
Below is a link to the help page that lists all category restrictions and links to give you specific information about each, why and what is needed to get an application submitted and approved:
- [Categories and products requiring approval|https://sellercentral.amazon.com/gp/help/help.html/?itemID=200333160&ref_=ag_200333160_h_r0_cont_sgsearch]
Also, to add, the brand restrictions could be in gated sales categories as well. Meaning that if you are approved to sell in the category, you may not be able to list a brand in it if Brand restricted too; requiring a brand application and approval before listing.