I came across this and thought it might be useful to others.
From Proofpoint:
https://www.google.com/url?sa=t&source=web&rct=j&url=http://tools.emergingthreats.net/docs/ETPro%2520Rule%2520Categories.pdf&ved=2ahUKEwi1s5eLlNfiAhWEVN8KHV0dDgkQFjABegQIBxAN&usg=AOvVaw0VeLizsYuRFk0ekY3FOOZz&cshid=1559903175835 (https://www.google.com/url?sa=t&source=web&rct=j&url=http://tools.emergingthreats.net/docs/ETPro%2520Rule%2520Categories.pdf&ved=2ahUKEwi1s5eLlNfiAhWEVN8KHV0dDgkQFjABegQIBxAN&usg=AOvVaw0VeLizsYuRFk0ekY3FOOZz&cshid=1559903175835)
http://tools.emergingthreats.net/docs/ETPro%20Rule%20Categories.pdf (http://tools.emergingthreats.net/docs/ETPro%20Rule%20Categories.pdf) ;)