I am sure it is defined by whatever your state wishes it to be. I do not think there is an entire answer that will fit all.
I can tell you that I was the alternate last year twice (Thanks Sonny!!!
).
All we did was chart who called a foul by position (L, C, T). And mostly talk to the head official and the girls basketball administrator, while we cracked jokes about anything that was discussed in our Thursday Meeting. I would be asked about my opinion of a situation or a rule by the head official (who is also the head clinician) and we would talk about the game. It was very laid back and more about the overall experience.
Also we were required to be totally dressed and ready to go. There was an official that got half dressed and was not ready to go and an official got hurt. So now we have to get dressed.
All of this might be different for your state or your role. They did ask that if something goofy happens to consult with the table last year before something happens. But otherwise it is just to be ready if someone gets hurt. I had my cell phone with me and people would text me about what associations was on my jacket (or not on my jacket). So it was an interesting experience and mostly the fun part of the weekend.
Peace