Often, SRFI authors write SRFIs that describe features that already exist in one or more Scheme implementations.  Sometimes, writing a portable implementation of a SRFI is difficult or even impossible, but the existing implementation is not licensed under the standard SRFI license.  In order to avoid requiring a completely from-scratch implementation in these and similar cases, I've added an explicit exception to the license section of the SRFI process.  In cases where the SRFI refers to an existing implementation, it's okay for that implementation to be under a different license at the editors' [editor's] discretion.  I'm specifically allowing the BSD 2- and 3-clause licenses.  Note that this applies only to existing implementations.  Sample implementations written from scratch specifically for the SRFI should still use the standard SRFI license.