~25 years ago ... I would have UML or, more classically, an ERD or even a flow chart The only diagrams I have seen in the last decade or two, though, have been

~25 years ago ... I would have UML or, more classically, an ERD or even a flow chart The only diagrams I have seen in the last decade or two, though, have been Visio (or a similar tool) meant to explain the product's functionality (including where and how it scales) to customers, PS folks, etc