What evidence to present for work-related AP?

my boss recently asked me to travel abroad for work related purposes.
What evidence will I need to present when I send a packet?
Proof of current employment?
Letter from my boss explaining my travels?

Thanks!

As you mentioned,

- Official Job letter explaining your position, rate of pay etc.
- Official letter from your boss/hr explaining that you must travel for work related purpose

These two will do it.