The Sonnet subsectioning is based on a uniform mesh indicated by the small dots in the project editor screen. The small dots are placed at the corners of a "cell". One or more cells are automatically combined together to create subsections. Cells may be square or rectangular (any aspect ratio), but must be the same over your entire circuit. The cell size is specified in the project editor in the Box page of the Circuit Settings dialog box (Circuit ⇒ Settings). The analysis solves for the current on each subsection. Since multiple cells are combined together into a single subsection, the number of subsections is usually considerably smaller than the number of cells. This is important because the analysis solves an N x N matrix where N is the number of subsections. A small reduction in the value of N results in a large reduction in analysis time and memory.
Care must be taken in combining the cells into subsections so that accuracy is not sacrificed. Em automatically places small subsections in critical areas where current density is changing rapidly, but allows larger subsections in less critical areas, where current density is smooth or changing slowly.
However, in some cases you may wish to modify the automatic algorithm because you want a faster, less accurate solution, or a slower, more accurate solution, than is provided by the automatic algorithm.
For a detailed discussion on subsectioning and how to modify the automatic algorithm, please see Subsectioning in the Sonnet User's Guide. There are also tips for selecting a good cell size in the Subsectioning chapter.