Some airlines limit your baggage based upon total weight instead of number of pieces. I'm not sure the context of this anymore. I believe KLM used to do this on intra-Europe itineraries. I believe Kenyan and Korean do on some itineraries. I'd guess their domestic itineraries, perhaps. In the case where a SkyTeam members itinerary uses the weight concept, the additional allowances apply. However, it has no applicability if your trip is on the piece concept.