Everybody probably has their own tolerance limits when it comes to flaming. One person's criticism is another's flame. I would say anything that constitutes a personal attack on another DIS member would be a flame. Unfortunately people who are very sensitive to criticism might think they are being flamed when they are not. Here's a completely made up example:
"Please don't flame me, but I cheated on my husband and I need advice. Should I tell him?"
Answer 1: "You're a slut." - That's a flame
Answer 2: "You really blew it. You need to tell him immediately before things get out of control." - That's not a flame.
Somewhere there's a fine line between strongly worded criticism and an outright flame.