The Vari-Quick shoulder strap harness on any regular Deuter backpack or child carrier can be replaced with the "VQ SL fit" harness, should the original harness not fit properly. Perhaps the padded straps are too long, too wide, or too far apart. The "VQ ACT Lite SL fit" harness is for using with the ACT Lite packs Likewise, the SL harness on any SL model backpack can be replaced by a "VQ standard fit" type for those with larger torsos. For ACT Lite packs use only the "VQ ACT Lite standard fit".