Outer chainring with two sizes
Front derailleurs are optimized for certain gaps between the rings and are limited to that - for example, the FD-6700 series from Shimano has a 16t front chainring difference (relating the outer and inner chainring size). There are also size constraints on the chainrings which determine the maximum and minimum size of chainrings you can use due to the shaping of the cage. If you're below the difference, within the chainring size limits and the derailleur clears the chainrings, you're good. So 52/42 should work. As should a 48 for the large ring on most road FD's. 

In the olden days, people had "half step gearing" which the front chainrings were close in size and they would crosschain (partially to get closer gearing and partially because old derailleurs couldn't shift 16t chainring differences). You can still do this now. 

52/39's are probably marked that way since a lot of people run that combination (and probably put it in the search bar for either chainring).