Ann is correct. However it is not as important to have the non-buffered board behind the image as it is for the non-buffered boards cut as the windows for the front mats. This is particularly of concern for Ilfochrome and Fuji Crystal Archive photos. The boards behind the photo do not contact the color of the paper (that is why the non-buffered board behind the photo is not as necessary). However, the concern comes in when the Azo dyes of the Ilfochrome or the color dyes of the the Fuji Crystal Archive photo paper contact the outer window mats. At least in the case of the Fuji Crystal Archive Photo paper, the pH is approximately 6. The buffered boards use a buffer that maintains the pH of a approximately 8.5 to 9. That is the concern for using these buffered boards in front of and contacting the edge of the color of the color paper and discoloring it in the 60 or so year life expectancy. The non-buffered boards will not cause this problem and with no buffers and a starting pH of approximately 7.5 will actually shift toward the pH of the photo and will not discolor the photo along the edge during it's life expectancy.
Here is some information I came across on the archivalmethods.com web site which may be of interest regarding buffered & un-buffered boards.
"Some suppliers of archival products are providing confusing information about the use of un-buffered boards and papers with photographic materials. Black-white and color photography (both conventional and digital output) should have mounts or enclosures made from buffered paper. This is the recommendation from the latest ISO document 18902, “Photographic Films, Papers Filing Enclosures”. The only items that need matting or storage in un-buffered papers are cyanotypes (architectural blue prints) and protein based textiles. ISO18902 is available at www.ansi.org,"