What does "retrying requests is not possible" mean?

[LPardue]

In the HTTP genart review, @theri writes:

Section 4.1.2

"Automatically retrying such requests is
not possible, unless this is otherwise permitted (e.g., idempotent
actions like GET, PUT, or DELETE)."
Retrying is "not possible" means it's prohibited? Is there a reference to the
exceptions to that rule, e.g., a list of actions that are considered
idempotent? If so, please provide a reference.

[MikeBishop]

Presumably a reference into SEMANTICS; I'll look for the right section to point to.