Hi Tudor, if you start with a complex surface and then boolean or trim away a large portion of it:






the original full surface is still there as the "underlying surface":




If you apply ShrinkTrimmedSrf to this, it will shrink down the underlying surface and cut out areas outside of the trim curve boundaries:



This can reduce surface complexity in cases like this and sometimes there can be problematic things like self intersections in other areas of the underlying surface that could interfere with some kinds of calculations.

Usually it is good to keep the full original surfaces because it makes repair operations by untrimming easier but if it's a very complex surface it can carry a lot of data size.

- Michael