Historically the indicators were used to check what was in the result field on your C-specification, so HI meant the result was higher than zero (positive), LO meant that the result was lower than zero (negative) and EQ meant the result was zero.
RPG has developed a lot over the years and the HI/LO/EQ indicators have been used for other operation codes and mean different things for different operation codes, for instance the indicators on a CHAIN operation are completely different to the indicators on a READ operation. There are many different uses for the indicators and I suggest you check in the RPG refernce books if you want more examples.
It doesn’t matter what the difference is. They shouldn’t be used at all in RPG IV (ILE RPG), so the difference is irrelevant. They are available for compatibility with old (non-RPG IV) programs.