well my understanding is that those who died in battle are like sakumo having unfinished business. (2nd hokage, 2nd mizukage, and the 2nd tsuchikage being some of those) and because they died in battle, most likely they would have unfinished business like sakumo. and it is because of that that they are unable to transcend from that plane. its like the pure world is a plane with two levels.thats kind of the opposite effect of what im saying i agree that there r multiple plains like the death gods stomach susanoos bottle and others but primarily there is the pure world where they go when they die and the impure world where everybody is alive when sakumo died his unfinished business brought him to that other plane and if he was still there i believe he could be unreachable but now he is not there and has moved on to the pure world which is where the jutsu summons them
level one is the plane where those who have unfinished business are: they cannot move on to level 2. level 2 being the place where souls have achieved peace and could transcend into something like heaven thereby rendering them unable to be summoned.