I think for most of the RSI & TOS members a flare is when the symptoms & pain, gets increased, from where it was at usually.

say someone one is at a 5 pain level usually, but they do something a little more than usual or if the weather changes drastically {usually to the cold /wet} and the pain level goes up to a 7/8 for a few days or weeks for some.

that's what I call a flare. Eventually the pain will decrease.