Peltier Module/TEG specs explainantion
QMax is the max amount of power it can pump at a specific average temperature across the device.

Delta T Max is the difference in temperature it will generate/need at the QMax condition.

However those numbers are just marketing numbers so you can compare unit to unit. The device can actually operate over a wide range on temperatures but the efficiency changes with average temperature. 

It's complicated. You need to look at the charts on the data sheet.