Phases are commonly used in projects to note milestones in the progression of a project, while task lists can be used to group different types of tasks together. It makes sense to be able to define a task list as either internal or external however the limitation that's currently in place where this capability is removed when a task list is associated to a phase, doesn't make sense. a phase such as delivery or discovery, or exicution isn't going to be internal or external however the tasks or task lists that are involved in that phase may be.