What does Reversed Participants mean?

The ReversedParticipants notice is just to let you know that the order of the participants might not be the same on both bookmakers.

For instance, on SBObet it might be:
Korea - Spain

And on 188bet it might be:
Spain - Korea

This could happen for instance if a match is played at neutral ground and no team is the home team. When the participants are reversed, you must (as you always should be doing) make sure that you back the correct players on the correct bookmaker, as it might be confusing when the participants do not have the same order at both bookmakers.