The first statement is required for all documents that might be
submitted for Standards Track publication. The primary motivation is
the the IETF retains change control, thus permitting augmenting the
original document to clarify or enhance the protocol defined by the
document.
The second statment is used when "republishing" standards produced by
other (non-IETF) standards organizations, industry consortia or
individual companies. These are typically published as
Informational RFCs, and does not require change control being ceded to
the IETF. Basically, these documents convey information for the
Internet community.
The third statement is used when the documents purpose is to provide
background information to educate and to facilitate discussions within
IETF groups, but can NOT be the basis for any IETF Working Group
activity. This is driven by the concern that unless a document author
agrees that it is subject to Section 10 of the Internet Standards
process (RFC 2026), it is impossible for the IETF to ascertain whether
or not there are any Intellectual Property rights issues with the
document.
I'll take an action to have "or be published as an RFC" added after "basis
for any IETF Working Group activity".