Epson GT-6500 EN - Manual (Page 44)

For example, if you specify 8 bits per pixel for each color, the scanner
produces 8 bits of data per pixel per color. This requires eight times
the data necessary to represent 1 bit/pixel/color. As the number of bits
per color changes, the data format for each byte of data changes as
shown below:
ESC D and other commands
Halftone processing with the ESC B command is only available when
the number of bits/color/pixel is set to 1 or 2.
Setting the data format to 2 bits/color/pixel disables halftoning modes
B and C on the GT-1000, GT-4000, GT-6000, GT-6500, ES-300C,
and ES-600C. (You can select halftoning modes B and C, but mode A
processing takes place instead.) Selecting a data format of 3
bits/color/pixel disables all halftoning modes on all scanner models.
2-6 Command Usage