You need both types of catalogues depending on whether your item is already listed on Amazon and whether you want to transfer new item data.

Creating a product catalogue

You create a product catalogue in the following cases:

• If the item you want to offer is not yet listed on Amazon.
• If you want to update existing item information (e.g. title, brand, bullet points).

Function: With the product catalogue, you transfer the merchant-independent item data to Amazon so that the item can be listed on Amazon.

Creating an offer catalogue

You must always create an offer catalogue in order to sell your item on Amazon.

• In addition to the product catalogue: You always need it to be able to sell the item, as it contains the merchant-specific data.
• If the item is already listed on Amazon: You create it to transfer your offer data (e.g. price, stock, delivery time, condition) and to attach yourself to an already existing listing.

In summary: With a product catalogue, you can list an item that does not yet exist on Amazon. But to actually be able to sell the item, you always need an offer catalogue in addition.