2.5 Variable References and #%top
Refers to a module-level or local binding, when id is not bound as a transformer (see Expansion). At run-time, the reference evaluates to the value in the location associated with the binding.
When the expander encounters an id that is not bound by a module-level or local binding, it converts the expression to (#%top . id) giving #%top the lexical context of the id; typically, that context refers to #%top. See also Expansion Steps.
|> (define x 10)|
|> (let ([x 5]) x)|
|> ((lambda (x) x) 2)|
|(#%top . id)|
Refers to a top-level definition that could bind id, even if id has a local binding in its context. Such references are disallowed anywhere within a module form. See also Expansion Steps for information on how the expander introduces #%top identifiers.
|> (define x 12)|
|> (let ([x 5]) (#%top . x))|