The PierToDo is used for a ToDo list to Pier. It is meant as a replacement for Epic Win. It is not that there's anything wrong with this iPhone app, it is just more convenient to have it on a site like this (dogfood - see Todo).
Children of PRStructure have some control of what their children are (and a child also has some control of what could be its parent) - this is through the messages canBeParentOf: and canBeChildOf:.
When first developing PRToDo, I found an error where only the admin user could add and remove tasks, this turned out to be the order of development (the PUSecurity didn't have these classes fully-working when it was created).