There have only been a few things I can remember specifically setting a boundary about with my therapists -- most boundaries come pretty naturally to me so they are generally not something I give much thought to. However, I absolutely put my foot down about relaxation exercises, mindfulness exercises, role-playing exercises. They are highly triggering for me for reasons we understand now after some exploration of what the issues were, and since we found other methods to accomplish the same goals, it worked out fine.