The distinctions are described in the documentation, found here:
https://slicer.readthedocs.io/en/latest/user_guide/modules/segmentstatistics.html
LM is the volume based on counting voxels in the label map representation. CS is the volume based on the region enclosed by the closed surface representation. These should always be similar, but can differ slightly due to smoothing applied to the surface representation. Which you should use for your purposes depends on whether you consider the label map or surface representations to be more reliable. In any case, it generally should make only very little difference which one you use.