The video tutorial describing the Heuristic Search functionality:
The default packer algorithm provides a decent packing for every UV map, but in most cases it won’t be the most dense one. If you want to obtain a very dense packing, you should enable the Heuristic Search functionality. In order to do that, enable the checkbox located in the header of the Heuristic Search subpanel:
When the Heuristic Search functionality is enabled, the packer will perform many packing iterations (slightly randomizing the process every time), saving the best result it could find. Using this technique you can improve the UV map packing density in a very short time.
The Search Time parameter specifies the time in seconds the plugin will spend on a heuristic search. After timeout is reached, the packer will stop and the best result will be applied to the UV map. If the time is set to 0, the packer will search for the best solution until you apply the result by pressing ESC.
The packer can significantly speed up a heuristic search by simultaneously iterating on all packing devices available in your system (CPU and all Cuda-enabled GPUs).
If you enable the Advanced Heuristic checkbox, the packer will use an advanced method during searching. It will examine a broader set of solutions when searching for the optimal one. This method is most useful when packing a limited number of islands – in such a case, it allows one to find a better solution than when using the non-advanced method. Enabling this option is not recommended when packing a UV map containing a greater number of islands.