It's an obsolete size. When the NMRA did the original work on the RP-25 recommended practice, they defined a series of proportional sized wheel contours, each intended to work with the common coarse standards of the different scales. These are defined here:

http://www.nmra.org/standards/sandrp/pdf/RP-25%202009.07.pdf

Code 72 was used for a while by people who anted a finer than common wheel in HO scale, although that use was not sanctioned by the NMRA because the wheel is too narrow to ride across the frog flangeways. More recently, work on the P:87 standard has encouraged some manufacturers to make exact scale width wheels, which are .064" wide, and therefore would be a "Code 64" wheel. Because of this, I suspect the Code 72 wheel is just going to fade away.

Dennis