Halliday & Matthiessen (1999: 409):
As a discourse develops, focus spaces are stacked one on top of another so that the most recent is always on top of the stack. The stack itself can thus be used [as] a record of progression through discourse time. Now the stack is always manipulated from the top: if a new focus space is to be added to the stack, it is pushed onto the top of the stack; and if an old one is to be removed, it is popped off the top of the stack.