I figure out that already, and post a thread in "Blame the Manual" section.
If you search starter in A7 manual you can't find anything about it.
I don't have much experience in scripting, when I study the examples in AUM, I found the starter function but can't search it in A7 Manual. So I download the A6.22 manual to know what is the purpose of starter. Then read A7 manual again and found _startup() is equivalent to that.