SharePoint Add-in is an identity principal just like a user, and it must be authenticated and authorized to use SharePoint resources. There are three authorization systems that an add-in can use. They are not mutually exclusive.
Three authorization systems and when to use them
- Low trust-
A provider-hosted SharePoint Add-in can register with Microsoft Azure Access Control Service (ACS), which issues an access token to the add-in that allows the add-in access to the resources in the SharePoint tenancy or farm on which the add-in is installed. Azure ACS is the trusted token issuer in an OAuth 2.0 Framework "flow" that includes SharePoint and the remote components of the add-in. Add-ins that use this system can be sold in the Office Store. The low-trust system is primarily intended for add-ins whose remote components are hosted in the cloud.
For more information about creating a SharePoint Add-in that uses the low-trust system, see Creating SharePoint Add-ins that use low-trust authorization.
- High trust - A provider-hosted add-in can establish trust with SharePoint by using digital certificates. The high-trust system is primarily intended for add-ins whose remote components are hosted on-premises. The add-in can be installed to a SharePoint farm that is not connected to the Internet. The add-in cannot be installed on SharePoint Online or sold in the Office Store.
For more information about creating a SharePoint Add-in that uses the high-trust system, see Creating SharePoint Add-ins that use high-trust authorization.
- Cross Domain Library - When the add-in's business logic is in JavaScript, you have the option of using the SharePoint cross-domain library either in place of, or as a supplement to, the low-trust and high-trust systems. The library is also intended for scenarios where the add-in has cloud-hosted components, but the customer's corporate firewall makes it difficult to use the low-trust system. The user's browser blocks scripts from other domains, but the library encapsulates a secure system for working around this restriction. Add-ins that use the library can be sold in the Office Store and can be installed to either SharePoint Online or on-premises SharePoint.
For more information about creating a SharePoint Add-in that uses the cross-domain library, see:
- Creating SharePoint Add-ins that use the cross-domain library
- Solving cross-domain problems in SharePoint Add-ins (blog post)
Hope it helps.