The existing interstitial link widget in Composer is based on a headline and an absolute or relative URL. This creates limitations in two areas.
1. When sharing across sites with different pathing structures, you are forced to link back to a single site even when the story has a valid site/path on that market.
For example, Chicago Tribune nation world stories (https://www.chicagotribune.com/nation-world/) that are shared on the Sun Sentinel nation world page (https://www.sun-sentinel.com/news/nationworld/) run into this problem: The URLs cannot be relative since the paths vary but if they are fixed, the user is taken to another site.
By using arc IDs instead of URLs, the links could resolve locally to the primary site/primary path by default.
Now, a URL option should still be available, but an arc ID option would be usable in many circumstances.
2. By having fixed URLs in links, we're forcing web view not AMP nor FBIA from articles on those platforms. We would rather a user who clicks on an interstitial link from AMP or FBIA stay on AMP or FBIA, where those are available.
The hope is that by using arc IDs, you can check in the back end and render on the original referring off-platform experience, continuing that user journey uninterrupted.