Translate a Complex Diagram or Schematic Into a Plain Description

Technical diagrams have conventions that are obvious to people trained in a domain and completely opaque to everyone else. Before you can read a schematic, flowchart, or system diagram, you need to know the reading method for that type. This explains the conventions, gives you the reading method, and translates what the diagram is showing into plain English.

I am looking at a diagram, schematic, or flowchart and cannot make sense of it. The diagram is of: {DIAGRAM_DESCRIPTION}. The context in which I encountered it: {CONTEXT}.

Translate it into a plain description:
1. What type of diagram this is: the category of diagram (flowchart, system architecture, circuit schematic, org chart, process map, or other) and the conventions for reading this type
2. How to read it: the starting point, direction, and method for reading this specific type of diagram
3. What it is showing: what the diagram describes in plain English, following the reading method
4. The key relationship: the most important relationship or dependency shown in the diagram, stated as a plain sentence
5. What is missing: any information a complete understanding would require that this diagram does not contain