>  Boucher, Virginia.  Interlibrary loan practices handbook.  Chicago : 
>    American Library Association, c1984.
>
>
>excerpts from the National Interlibrary Loan Code (1980), a copy of which
>is found in an appendix in the book above.
>
>II. Purpose
>
>The purpose of interlibrary loan as defined in this code is to obtain, for
>research and serious study, library material not available through local,
>state, or regional libraries.
>
>IV. Responsibilities of Borrowing Libraries
>
>    F. The borrowing library should carefully screen all requests for loans
>       and reject any that do not conform to this code.
>
>
[...]

Here is a quote from another interlibrary loan policy in the same
book:

---begin--

Model Interlibrary Loan Code for Regional, State, Local, or Other Special
Groups of Libraries

[...]
II. Purpose

The purpose of interlibrary loan as defined in this code is to obtain
library material not avalable in the local library.
[..]

III. Scope

Under the terms of this agreement, it is permissible to request on
interlibrary loan any type of library material [if they want libraries
can make an exception for items like rare books, etc - cmk]

[...]

Any member of the borrowing library's clientele should
be eligible for interlibrary loan.

-- end --

So, when does each policy apply?

The preface of the model code says:

"The 'Model Interlibrary Loan Code for Regional, State, Local, or
Other Special Groups of Libraries' is intended to provide guidelines
for any group of libraries interested in developing an interlibary
loan code to met special needs. The Model Code, while complementing
the "National Interlibrary Loan Code, 1980" allows libraries more
flexibility and creativity in satisfying interlibrary loan needs in a
specific situation."

Here is the interlibrary for the National Interlibrary Loan Code, 1980:

"Interlibrary loan is essential to the vitality of libraries of all
types and sizes and is a means by which a wide range of material can
be made available to users. This code is designed primarily to
regulate lending relations between research libraries and between
libraries operating outside networks or consortia. It is recognized
that through specific agreements, libraries organized geographically,
by mutual subject interest, or other bases will have developed code of
their own. It is not the intent of this code to prescribe the nature
of interlibrary lending under such arrangements. (See 'Model
Interlibrary Loan Code for Regional, State, Local, or Other Special
Groups of Libraries')."

Is it the leading or the borrowing library's resonsibility
to enforce the code?

The more restrictive National Interlibrary Loan Code says:

IV.F "The borrowing library should carefully screen all request for
loans and reject any that do not conform to this code."

V.A "The decision to loan material is at the discretion of the lending
library. Each library is encouraged, however, to interpret as
generously as possible its own lending policy with due consideration
to the interest of its primary clientele."