There's now a "Role" column in
<https://registry.scheme.org/#hash-bang-syntax>.

As far as I can tell, there are three kinds of #! syntax:

- Directives. These change the lexical syntax used to read the rest of
the code, e.g. #!r6rs or #!fold-case.

- Objects. These evaluate to a unique, symbol-like object with a special
purpose, e.g. #!eof and #!void.

- Keywords like #!optional and #!rest that are used as sentinel values
in macros to indicate that a particular section of the code begins. E.g.
#!optional, #!rest in lambda lists.