I'm not sure if it's a North American thing or an Oil and Gas thing, I never came across one in the UK nor my first few jobs in Canada, but since I've moved to Alberta and Oil and Gas its one of the most common things I hear.
It's basically a matrix, the shutdown key is the safety circuits.
Imagine a spreadsheet, the left hand column would have all the safety inputs listed and the top row across would have all the safety outputs, following the columns down from each output there would be a code against any row where the input at the left had an effect, if the input has no effect on an output its blank.
Code could be SD = Shutdown, FC = Forced Closed, etc.
There's also a cause and Effect diagram which are non safety items doing the same thing.
Its a visual of the safety system. Apparently its good, I disagree, hate them.