How the external system can access a Appian interface?

External system can access the Appian interface via

1. Embedded Interface
2. Portals

Embedded Interface :

An embedded interface is an Appian interface that runs inside someone else's web page. The external page loads a small Appian script tag, and the task, action or report renders in place, so the user never leaves that page or opens an Appian site. With a custom theme the embedded part can match the host page's own styling, so it is not obvious that Appian is behind it.

What can be embedded this way:

1. Tasks
2. Actions
3. Related actions
4. Reports (including task reports)
5. Record views

Learn more about it here

Appian Portals:

It connects external users to the information and processes that you already have in Appian. With Appian Portals, you can use low-code to build public websites that are conveniently connected to your Appian applications.

Public users can use portals to submit forms, file claims, view documents and information, and more. All without requiring them to log in.

Configuring interface in portal

Create a new portal object > Click on add page -> you can add the interface which you want to expose to the external system