Created during the vbsp process only if a leak is present, it begins at an entity close to the leak and the follows the raytracing steps it took to find its way into void. Often not straight and with a meandering path, it may take some precise following to find the area where the leak is present, it is more a line threaded through the leak rather than a line that points directly to it.
If multiple leaks are present only one will be found and documented, it may take multiple compilation attempts and the corresponding .pts files to find all leaks in a map. Hammer can automatically load the the .pts file for the map and the compile. A map's or compile's .pts file is useless to any other map or compile.