What is the difference between pin 2 and pin 3 hot?

Balanced inputs and outputs that utilize the XLR type of connector look for either pin 2 or pin 3 to be the 'hot' signal, where the other 'cold' signal will be polarity reversed in the balancing circuit. Pin 2 hot is the most common convention now, although pin 3 was quite common in the past. In most cases it really doesn't matter which pin is not when going to the unit and back as there will be no polarity reversal to worry about since it's the same going in and out.