Advanced settings allow you to change Classic Commerce defaults and integrate with the API. Go to: Classic Commerce > Settings > Advanced.
The REST API settings are for developers to use. You can enable it to access store data from outside ClassicPress by creating API keys to work with the Classic Commerce REST API.
A Webhook is an event notification sent to a URL of your choice. Users can configure them to trigger events on one site to invoke behavior on another. More details are available on the Webhooks page.
The Legacy API is no longer maintained, but can be enabled in this section if necessary.