Midtone Contrast:
The midtone contrast actions are designed to increase contrast
only in the midtones and thus protecting the complete blacks
and complete whites of your photograph. The actions are setup
in a way that the topmost is least effective and the bottom one is
most effective, they also utilize two different ways,
the first five will use a combination of unsharp mask with layer settings
and the last five will use a high pass filter in combination with layer
settings. Overall the unsharp mask actions are less effective than the
high pass ones.
Which one you have to use on your photographs will vary
on a image to image basis.
Experimenting is key here !