Epson GT-6500 EN - Manual (Page 134)

Explanation:
This command specifies the gamma correction setting for the output
device.
The gamma correction refers to the gamma correction table stored in
the scanner’s internal memory, and adjusts the conversion ratio
between the input intensity of the original image and the output image
data values.
The various predefined mode settings have the following
characteristics.
CRT Display A (for bi-level data)
This setting is suitable for CRT display types that are incapable of
displaying different tones of gray or colors. For example, this setting
is suitable for displays that can produce only 8 or 16 colors.
CRT Display B (for multilevel data)
This setting is suitable for high performance CRT displays that can
display different tones of gray or colors. For example, this mode is
suitable for displays that can show more than 16 colors.
3-40 Command Reference