It's just a diff (long tailed) pair wired as a schmitt (one bipolar diff in coming from the opposite bipolar's collector out.) Reference information is located here. If all that's needed is which bipolar is on or off, with respect to Vo, then it's kind of obvious. Clearly if Vo is near 15 V then T2 is off and T1 is on. If Vo is closer to ground (it won't reach ground, but perhaps a few volts above it), then T2 is on and T1 is off. How could it be otherwise? Out of which field this question comes? I've no idea. The answer is obvious.
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