a320
 

if you look at the air condition on the overhead panel there is a start under a green arrow, what's it's purpose?

The lines on the overhead panel are a schematic of the bleed system. The green lines with “Start” show where the bleed supply for starting each engine is taken from. You could then see, for example, that if the crossbleed is closed, you wouldn’t be able to start engine 2.

Yup, you can also trace out the flow in the ECAM. Very useful.