Question about State machine of batch task and instance

As what is given in trace_201708.md, we found that both task and instance all have status of "Waiting". and what is declared is:
task -> Waiting: A task in not initialized yet
instance -> Waiting: The instance can't run because some of its dependencies have not finished
IIUC, if a instance's status is "Waiting", we can be sure that there is some dependency among tasks that has not been satisfied. so whenever task's status is no longer "Waiting", its instances' status can change.
so the "Waiting" tasks mean that they are waiting for other tasks finishing ?

In addition, I found that some instance reboot after it arrived at "Failed" status, but others not. Is there any mechanism for judging whether a instance should reboot?

I will appreciate if someone could help.