+5V Auto Selector

Hey guys,

I want to build my own Arduino Leonardo based on the schematic that I found here.
But I did not understand how the 5V auto selector works. In schematic it looks like (T1) PMOS enhanced mode transistor plays a key role in the whole process, but I think it should be PMOS depletion transistor.
I will highly appreciate if somebody could explain this mechanism to me.

All the best,