[Frontier Math is composed of] 25% T1 = IMO/undergrad style problems, 50% T2 = grad/qualifying exam style porblems, 25% T3 = early researcher problems
The comment was responding to a claim that Terence Tao said he could only solve a small percentage of questions, but Terence was only sent the T3 questions.
From an apparent author on reddit:
The comment was responding to a claim that Terence Tao said he could only solve a small percentage of questions, but Terence was only sent the T3 questions.