Jesus often uses code words so that sayings have more than one meaning. All the meanings can be true. The encoded (literal) form makes (some) sense and the decoded form makes sense. In general:

• Noun words can be code words. Pronouns can be ambiguous.
• Verb words are carefully chosen to work in both contexts.
• Play on words, Spoonerisms, etc., are sometimes used as the code word meaning. These may or may not be commutative.

In the security field, by definition, a code word is a word that needs to be substituted with another word (e.g., using a code book) to determine the meaning. By definition, in the security field, a secret code is a code whose meaning is to be obscured to anyone without the code book.

So Jesus, by his own declaration, uses secret codes. However, these codes appear to provide only authentication information (and warnings about certain heresies) but no special salvation knowledge - which is always is clear text.

More: Pronouns

More: Parables and secret codes used and explained by Jesus

Jesus uses, by his own explanation, (secret) code words where nouns refer to other words/meanings and not necessarily the literal meaning(s). In terms of "heaven" there appear to be three related code words or phrases.

Translators have translated plural "heavens" as singular "heaven" and then used opinion to make up reasons for this. Those reasons do not appear to fit how the words are used by Jesus nor how they are used in the LXX (Septuagint).

Note that Jesus knows the code words and appears to use them consistently. Others, including Matthew (who appears to have written down what Jesus says), do not appear to know about these code words and their associated meanings. Thus, others do not use those code words consistently.

More: Parables and secret codes used and explained by Jesus

More: Sets: Mutually exclusive and collectively exhaustive

The ancient Greek word "οὐρά" ≈ "tail" of an animal such as a lion, dog, etc. In military terms, it could be the rear-guard of a marching army. The word appears five times in three verses in the GNT (Greek New Testament) in Revelation and in the LXX.

What might the symbolic meaning of the "tail" be? The "head" is usually considered opposite to the "tail". In English, the terms "heads" and "tails" refer to opposite sides of a coin.
Saying (of snakes, etc.): Heads I win. Tails you lose.
Saying: I can't make heads or tails of it.

If Jesus is the "head", then who or what is the "tail"? The ancient Greek word for "tail" is related to the (vulgar) word "ὄρρος" ≈ "backside".

More: Minding the tail of the sky