5.6.1 Substitution function syntax

[Kees van den Doel]

Referring to https://lilypond.org/doc/v2.22/Documentation/notation/substitution-function-syntax

I don't understand this sentence explanating "...music...":

>normal LilyPond input, using $ (in places where only LilyPond constructs are allowed) or # (to use it as a Scheme value or music function argument or music inside of music >lists) to reference arguments (eg. ‘#arg1’).

The next section in the docs with examples has no example of use of '$'. When should I use $?

Thanks,

Kees