Use the context view to reference external systems’ documentation or to provide references to the parts of your arc42 documentation containing appropriate information.

If an external system/interface is especially important or risky for your architecture, it’s sometimes adequate to copy parts of this interface document into your own arc42…

War Story (from Gernot)

In one customer project we had to poll an external system for user-credentials: No user could work with our system when this special interface was down. One day the incredible happened - we had a fatal production bug caused by exactly this interface. Too bad the documentation of that (external) system was located on a network share… that was accidentally down at the same time - so no developer could access the documentation that was essential to tackle the production bug…