Sarcasm is very hard to get across when using the internet, so typing SARC-INT will make sure people get your hint from the get-go, and saves you precious time explaining that you were actually being sarcastic
Person 1 - "I love that hat ur wearing in ur prof pic."
Person 2 - "Really?"
Person 1 - "I was being Sarcastic."
Person 2 - "Ahh right, yeah lol I was forced to wear it."
Person 1 - "I love that hat ur wearing in ur prof pic. (SARC-INT)
Person 2 - "Yeah tell me about it lol."