It means nothing. In the old days there were several rim sizes that would all use tires of the same outside diameter; 700a, 700b, 700c. Currently nearly all road bikes (and 29er mountain bikes) use the 700c wheels with different widths of tires. You still see two different 650 rims still in use; 650b and 650c.

The 700 refered to the outside diameter of the original tire that was used on that size of rim. The bead diameter of the 700c rim is actually 622mm.