Epson Expression 1680 Reference Manual page 18

To change the tone curve back to its default, select Linear from the
Tone Curve Name list.
To add your settings to the Tone Curve Name list, type a name
for the settings in the list, then click Save.
Output values
These are the current values for output data in accordance with
the current locations of the points inside the tone curve editor.
These values change when you drag points in the tone curve
editor, or you can enter in values here to change the shape of the
tone curve. Each value can be adjusted within a range of 0 to 255.
Note:
Each output value name has a check box to the left of it. Highlight
and Shadow are always selected, but you can select or clear 1/4
Tone, Midtone, and 3/4 Tone by clicking their check boxes.
When a value's check box is clear, its point on the tone curve editor
automatically shifts to smooth out the tone curve when one of the
other points is moved.
When a value's check box is selected, it means that its point is fixed
and it will not shift when other points are moved.
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Optimizing Scanning