Would it be worth having two columns with "can have milestone", "can have subproject"?
Mainly to illustrate that the (main) parent cannot have milestones and that milestones cannot be further sub-divided.
Topic on Talk:Phabricator/Creating and renaming projects/Project types
That sounds useful. But I thought the parent could have milestones?
A parent can. but not the main/topmost parent per Project management ("Note this potentially important but non-obvious consequence of the membership rules: Parent projects can't specify members, and milestones can't specify members. So a milestone's parent must itself be a sub-project, in order for that sub-project's members to be inherited by the milestone (and by the parent).")
Ok. so after some confusion. That issue only relates to inheritance of membership. With that clarified I'm less sure about the need for the new columns.