'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)

(References: Apocalypse Explained 400)

2418. 'Do not halt in all the plain' means he was not to linger over any one of these. This is clear from the meaning of 'a plain' as every aspect of doctrine, dealt with immediately below. What not lingering over any one of them entails will be stated at verse 26, where Lot's wife is referred to as looking back behind him. That 'a plain' in the Word means all aspects of that doctrine is clear in Jeremiah,

He who lays waste will come to every city, and no city will escape; and the valley will perish, and the plain will be destroyed. Jeremiah 48:8.

'City' stands for false doctrinal teaching, 'the plain' for all aspects of that doctrine. In John,

When the thousand years have come to an end Satan will be loosed from his prison, and will come out to deceive the nations which are at the four corners of the earth, Gog and Magog, to gather them for battle; their number is like the sand of the sea. They went up therefore over the whole plain of the earth, and surrounded the camp of the saints; but fire came down from God out of heaven and consumed them. Revelation 20:7-9.

Here 'Gog and Magog' stands for people whose worship was external devoid of internal and so had become idolatrous, 1151. 'The plain of the earth' stands for the Church's matters of doctrine which they lay waste, 'the camp of the saints' for goods that flow from love and charity. 'Consumed by fire from God out of heaven' is similar in meaning to that regarding the men of Sodom and Gomorrah, in verse 24. Also in Jeremiah 33:13 matters of doctrine regarding charity are called 'cities of the mountain' and matters of doctrine regarding faith 'cities of the plain'.