- Short answer
Both scope/context (arc42 section 3) and building block view (arc42 section 5) provide adequate room for external interfaces.
- Longer answer
How and where you describe external interfaces depends on several factors:
- If your system provides external interfaces to external consumers and you and your team can decide about any details of these interfaces - then you have to care for their documentation.
- Do you need to provide external systems with information on how to use your interfaces? You could provide example code and other interface documentation, but need to make sure this information is easily accessible to the external stakeholders.
- Do you just provide data at your external interfaces, and no services? Then you might stick to just a documentation of data formats (and maybe semantics).
- In case you use interfaces provided by external systems, make sure their documentation is easily accessible by your development and maintenance teams. Include references in the arc42 context or building block view.