There is no one list of what a Brand Ambassador does. It really depends on the company that the Brand Ambassador is representing. If you want to be a good Brand Ambassador and get a job working for a company, then you should choose an industry and develop your skill set and be a great Brand Ambassador for the industry you choose.
This can be much more lucrative than just trying to be a general Brand Ambassador. The reason is that a good Brand Ambassador is going to network and know the major players in the industry they are working for. If you continue to focus on one industry and develop your skills around that industry, you are going to find that you have companies in the industry competing to hire you as their Brand Ambassador eventually.
Case in point, I have a friend that is a great sales person in one particular niche industry in manufacturing. It is nothing flashy, but he commands a very high salary, plus big commissions, plus a car and other perks because he knows everyone in the industry. I saw him try to move temporarily to another industry and he was not nearly as effective. He is great in his focused area.
So if you want to get a job as a Brand Ambassador and there is an Industry you really love or already know well, consider focusing and you will be paid better and have more opportunities.