UML stands for Unified Modeling Language. It's a standardized set of notation and symbols for modeling workflows, data diagrams, etc. It's different than flowcharting because it covers details - though still high level - of how a process is supposed to work; the inputs, outputs and other interfaces; messages that are sent from one process to another.
Being a programmer, a geek in general, and a visual person means I really like diagrams and charts. I'm having a great time right now creating a chart showing the relationships between Spark rpg setting creation elements. A version of UML for gaming is really intriguing idea, but I'm not sure how practical it would be. From a system design perspective, it might be useful just to get the feel for how the system flows and identify any potential issues. There are a number of things in the UML standard that could be taken as is, but some of the notations and symbols would need to be changed I think, particularly several of the structural things and notations. Maybe someday I'll tinker around with doing a straight UML diagram of Fate Core combat or something just to see how it all fits together.