PFD - Prepared for Dyeing - means that the fabric - or garment - is specially made to be dyed. PFD fabrics have been desized, scoured, and fully bleached, and they have not been finished or processed with optical brighteners or softeners that can interfere with dye uptake, resulting in fabrics that lack saturated color or are not evenly dyed.
Bella 97 PFD has been very popular for many years because the "bleached white" isn't quite as "WHITE" as the Bleached Bella 98. Bella 98 has been finished with optical brighteners to make it brighter, i.e., more "white". It has also been finished to prohibit the absorption of dye, including any excess dye that might be released when the fabric is washed or wet.
Bella 97 has not been finished that way, it has been purposely manufactured to maximize the absorption of the dye, whether that dye is in a dye bath or in a washing machine. It is acting as a magnet for the dye because that is what it is supposed to do.
While it is not necessary to know these terms, they might help you decide which fabrics to use.
Desizing - that is the process of removing the "size" material from the warp yarns after the fabric has been woven. Sizing is used in the weaving process to strengthen the warp - lengthwise - yarns to reduce breakage.
Scouring - a chemical washing process that removes natural waxes, soil and any non-fibrous impurities (e.g., seed fragments). Scouring is a prerequisite to most other finishing processes.
Bleaching - even after scouring, even the most naturally white cotton fibers are yellowish so bleaching is often required. It improves the whiteness by removing the natural coloration and trace impurities from the cotton. The degree of bleaching necessary is determined by the required - and desired - whiteness of the fabric.
The next two steps are not used for PFD fabrics because they generally inhibit the absorption of the dye.
Optical Brighteners - these are synthetic chemicals used to make clothing appear whiter and brighter, and thus cleaner looking. Optical brighteners are frequently added to liquid and powder laundry detergents. They are the modern day replacement for the decades-old practice of bluing - adding small amounts of blue dye to fabric to make it appear whiter.
Softeners - finished textiles are given a softening treatment to give the fabric a desired "hand" or feel.
So what does this mean for non-dyeing quilters?
Bella 97 PFD should only be used when dyeing fabric. Using it in place of Bella 98 or Bella 200 can result in color transfer, especially when used with fabrics with saturated color.