The credits are for reducing HFC-23 emissions that arise in the process of making a product called HCFC-22. HCFC-22 is an ozone-depleting substance that is scheduled to be out of production under the Montreal Protocol in the developed world by the end of 2010 and with a later phase-out. It's a highly potent greenhouse gas.
On November 27th, 2007. See this statement in context.