'A plain' signifies the extremes of the Word regarding doctrine, and also the extremes of the church, or people in extremes of truths and goods. It also signifies good and truth in the natural self, because 'the people who dwell in plains,' or 'beneath mountains and hills' are in the extreme heaven. In Genesis 19:17, this signifies all doctrinal matters.

In Genesis 11:2, this signifies that their worship became more impure and profane. (Arcana Coelestia 1292)

In Jeremiah 21:13, this signifies the faith in which is no charity. (Arcana Coelestia 4715) Plain and valley signifies the ultimates of the Word. (Apocalypse Explained 411) Plain signifies good and truth in the natural man, because people in the First Heaven dwell in plain or valley. (Apocalypse Explained 449)

(Odkazy: Apocalypse Explained 400)

211. From all this it can be seen what the form of heaven is, namely, that it is the most perfect of all in the inmost heaven; in the middle heaven it is also perfect, but in a lower degree, and in the outmost heaven in a degree still lower; also that the form of one heaven has its permanent existence from another by means of influx from the Lord. But what communication by influx is cannot be understood unless it is known what degrees of height are, and how they differ from degrees of length and breadth. What these different degrees are may be seen above (38).