Halliday & Matthiessen (1999: 514):
What is said to exist may be an entity, something that persists through time, like there’s a letter for you; but it may also, in many languages, be a happening, as in there was a fight. Here we have something that could alternatively be construed as a material process (people were fighting), which suggests that “existential” processes are another intermediate type, something between the relational and material.