But it is a rule of writing said differently. The writing rule is "The first use of the word, particular when defined, sets the terms for that word in the rest of the writing." That doesn't mean it can't be used later on as a metaphor, but to use it as a metaphor, one must first understand the meaning of the word.
I also have no formal training. Sorry in advance for my rambling.I hope it adds to the discussion.
I would agree with what was offered above . Since we are not to lean on our own understanding it would be more like comparing the spiritual "understanding" that God applies to a word (singular ) as the one teacher, to the same spiritual "understanding" presented in a poetic form.
Therefore we compare understanding to understanding.
Even adding to a word designed to be used as singular, making it plural can destroy the authors intention.
Personal poems which I consider the Bible to be more like, need the understanding intended for them. It can be like abstract art that give the understanding of the artist and without his understanding ..its anyone’s guess .
It is I believe, one of the reasons he gives us his loving commandment in Deuteronomy below to help and comfort us from false prophets or false apostles who use their own understanding to draw men unto themselves.Changing the spiritual meaning he applies to a single word can change the direction of the good news.
Now therefore hearken, O Israel, unto the statutes and unto the judgments, which “I” teach you, for to do them, that ye may live, and go in and possess the land which the LORD God of your fathers giveth you. Ye shall not add unto the word (singular) which I command you, neither shall ye diminish ought from it(singular), that ye may keep the commandments(plural) of the LORD your God which I command you. Deu 4:1
In the above when he is informing us how we can hear Him who has no form. He uses statues, defined as the limit or law of something prescribed, due. Therefore unto or towards judgments, which would be the secret decisions of God's will. When I say secret decisions we must remember he hides the spiritual meaning in parable from natural man. We are informed in that way without parable Christ, the anointing Holy Spirit of God spoke not.
The word
apostle comes to mind as an example . Adding another meaning to that word has caused much confusion among Christianity. In many cases the word apostle has been said it only applies to the New Testament. The Catholic as an example will argue that point to death. It would make their kind of foundation crumble if they did apply God interpretation rather than that of their fathers oral traditions as commandments of men ... Seeing they have added many new meaning to that word so that they can come up with the idea of Apostolic succession, beginning with Peter. . If we would apply the word apostle which simply means sent one to Moses or Abraham as men sent by God... it would destroy their foundation as commandments of men as a private source of faith.. The verse in Deuteronomy applies . This is unlike the warning at the end of prophecy which is in respect to adding more words (plural) to the now perfect/complte Book that we had in apart up, until Revelation.
Song of Songs is a more extreme example of his poetic language or the book of Revelations which is loaded with parables that must be sought out to find the spiritual understanding.
In one sense the whole Bible is God’s love poem to His bride. Likened to Song of Songs, a picture of God singing to us His bride.
Below God is pictured as singing rejoicing over his bride the church.
Zephaniah 3:17 The LORD thy God in the midst of thee is mighty; he will save, he will rejoice over thee with joy; he will rest in his love, he will joy over thee with singing.
Seeing God hides His spiritual understanding from natural man and reveals the understanding of a parable as a honor to His kingdom of priest.. the privilege to search it out . Which all believers are considered Priest from his kingdom ,not of this world. He is considered the King of kings(believers) throughout the scriptures from that standpoint.
I think the spiritual word is the understanding that God attaches to it. Like in the parable below also called a proverb or a by word seeing they all come the same Hebrew word. Personally the word parable works the best for me.
These are also proverbs (parables)
of Solomon, which the men of Hezekiah king of Judah copied out.It is the glory of God to conceal a thing: but the “honour of kings” is to search out a matter. Pro 25:1-2
There I believe he is referring to kings as His priests that he sends to bring the gospel as ambassadors for Christ sent from a foreign land in respect to the new Jerusalem, which is typified as His bride the church..It is from that vantage point that Christians have their born again birth rights kept from moths and rust.The corruptible things
The heaven for height, and the earth for depth, and the heart of kings is unsearchable. Pro 25:4
Because the heart of God is unsearchable without his revelation, as His interpretation. I believe it helps us understand why he informs us our hearts are something we cannot know . Seeing they are simply a judgement hall that either accuses us or excuses us. We therefore are not the standard.His interpretation the bible is.
Pro 25:4 Take away the dross from the silver, and there shall come forth a vessel for the finer.
In that verse he uses silver to identify His wife the church as it is cleaned by the water of the word the gospel of our salvation.
Pro 25:5 Take away the wicked from before the king, and his throne shall be established in righteousness.
The king there is twofold with Christ, as our King of kings
Pro 25:6 Put not forth thyself in the presence of the king, and stand not in the place of great men: 
indeed my brethren my brother
