Especially external interfaces might occur several times within the building block hierarchically - how can I avoid to document or specify them at several locations redundantly?

See the diagram below - the interface X (marked with red circles) occurs three times in the hypothetical system shown there.

Handle such situations in the following manner:

  1. In the context diagram, give the interface an appropriate name and briefly explain its business relevance or significance in just a few words.
  2. Describe the interface in detail at the level where it is actually handled (e.g. the service is implemented or data is consumed). In the diagram above, this would be whitebox “B”.
  3. At all other occurrences (especially in the context view), add references to the detailed description. In the scenario above, such references should occur in the context and the level-1 whitebox.