https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/get-started-creating-provider-hosted-sharepoint-add-ins
- They include a web application, service, or database that is hosted externally from the SharePoint farm or SharePoint Online subscription. They may also include SharePoint components. You can host the external components on any web-hosting stack, including the LAMP (Linux, Apache, MySQL, and PHP) stack.
- The custom business logic in the add-in has to run on either the external components or in JavaScript on custom SharePoint pages.
Can use the CSOM – client side object model
using Microsoft.SharePoint.Client;
