I looked into this and can verify that there is no visible effect when changing the decimation rate. The property "Decimation rate" is correctly used inside ShowSegmentationAsSurface::ConvertBinaryImageToSurface as reductionRate. It is then used as an argument for ImageToSurfaceFilter::SetTargetReduction. The brief description of this parameter is:

I tested this and the smoothing works but as stated in the description it should be made more obvious what to do.
As @kislinsk wrote we will not tackle this issue here but instead remove the options from the preferences. However, this is more about the context menu action "Create smoothed polygon model". Should the whole action be removed? Or are we only talking about the additional preferences for "Smoothed surface creation"? If only the preferences should be removed we still need to adapt the checklist to make it more clear how a smoothed polygon should be created.

A short but clear description of the interaction types is given in the MITK StdMultiWidget Editor help page (F1 on "StandardDisplay"):

I changed the strings according to the suggestions in this task.
However, your last comment only stated to rename the "Clear Segmentation" button.

I updated the "Checklist Segmentation – 3D Segmentation_EN":

• updated tool-image (no region growing, new icon for Picking and GrowCut)
• removed region growing test section, changed order of tool sections
• updated Otsu-tool GUI image
• added description of "Transfer ..." checkboxes
• modified description of "GrowCut"-tool

I looked into this and I was confused: the used QmitkRenderWindowDataStorageTreeModel already contains a node predicate for filtering out the helper nodes. When creating the mxnmultiwidget (or a new render window) there are no plane geometries (the crosshair) being added to the storage model.

A simple context-menu was added in {D726}. Needs to be reworked to allow more actions.

{D727} was opened in addition to {D722} to fix some bugs, make the code cleaner.

The geometry-comparison has been fixed. Slice interpolation needs to be tested and probably tackled as well.

Pixel information has been removed. To add a specific view for pixel information, see T29324: Create view for pixel information.