Relation between programming languages requiring declaration of variables before use and using the token class $\text{id}$ while parsing I was going through the text Compilers: Principles, Techniques and Tools by Ullman et. al. where I came across the following excerpt.

Example 4.11. Consider the abstract language $L_1 = \text{ { $wcw$ | $w$ is in $(a|b)^*$}}$. $L_1$ consists of all words composed of a repeated string of $a$'s and $b$'s separated by a $c$, such as $aabcaab$. It can be proven this language is not context free. This language abstracts the problem of checking that identifiers are declared before their use in a program. That is, the first $w$ in $wcw$ represents the declaration of an identifier $w$. The second $w$ represents its use. While it is beyond the scope of this book to prove it, the non-context-freedom of $L_1$, directly implies the non-context-freedom of programming languages like $\text{Algol}$ and $\text{Pascal}$, which require declaration of identifiers before their use, and which allow identifiers of arbitrary length.
For this reason, a grammar for the syntax of $\text{Algol}$ or $\text{Pascal}$ does not specify the characters in an identifier. Instead, all identifiers are represented by a token such as $\text{id}$ in the grammar. In a compiler for such a language, the semantic analysis phase checks that identifiers have been declared before their use. □

I can understand that since $\text{Algol}$ and $\text{Pascal}$, require declaration of identifiers before their use we cannot check this property using context free grammar. But what is the connection of this with the point of &quot;allowing identifiers of arbitrary length&quot;?
Moreover the authors add, that instead of using the characters in an identifier, identifiers are represented by the token class $\text{id}$. That the identifiers are represented by the token class $\text{id}$, was known to me as a fact, but I did not quite know what is the significance of this as far as the explanation in the example of the text is concerned.
Please explain me.
In order for their example to work, the authors need identifiers to be of unlimited length. This is because the language
$$ \{ wcw : w \in \{a,b\}^*, |w| \leq n \} $$
is context-free (indeed, regular).
The syntax of a language like Pascal or Algol is context-free. This accomplished by waiving the requirement that an identifier be declared before its usage; this will be checked on-the-fly by the parser. This idea is implemented by representing identifiers as a single token in the grammar of Pascal or Algol.