Do you mean a pass-through permit?
If you wanted a Little Yosemite Valley permit to do Half Dome or Clouds Rest but LYV is full, you can get a pass-through permit. It enables you to enter via Happy Isle or Glacier Point and hike to LYV. You cannot stay at LYV with a pass-through permit. You have to go at least a mile past LYV, and camp on either the Sunrise trail or the Merced Lake trail. Most people planning to do Half Dome or Clouds Rest use pass-through permits to camp near Sunrise Creek at the John Muir Trail / Clouds Rest trail intersection.
See Happy Isles to Sunrise/Merced Lake (pass-through) under the Yosemite Valley section at
http://www.nps.gov/yose/planyourvisit/trailheads.htm.