The "difference" these days is Marketing, not physical, and you can use them interchangeably as long as they are made for the same number of cogs. A 9-speed road chain and a 9-speed MTB chain are the same except for the labeling.

At one time Shimano made HG and IG 7/8-speed chains that did have slightly different shapes with the HG recommended for road and the IG recommended for MTB use. The overall width and pin lengths were identical and they could be used interchangably with no obvious performance degredation. I used IG90 chains on my road bikes for years and they shifted fine.