Three required values, eight optional: which one do you need and when?
Amazon asks for more fields than any other platform. Only three of them are required, though: the LWA client ID, the LWA client secret and a refresh token. All three live under Seller Central → Apps & Services → Develop Apps, where you generate the refresh token by self-authorising your own app. The rest are filled in per scenario and stay empty for most sellers.
One distinction saves time: the merchant token is not needed to pull orders, but it is needed to push stock and prices. So if you are only tracking orders you can leave it empty; if you are turning on stock sync you have to fetch it from Seller Central → Settings → Account Info. This is the most common cause of "orders arrive but stock does not go out".