Epson GT-6500 EN - Manual (Page 30)

Data form definition
Data format
The data format function determines how many bits are used to
represent one pixel. Increasing the bits increases the number of shades
and intensity that can be expressed. However, the data required to
represent an image is also increased. For example, to scan an A4 size
image using 8 bits per pixel at 300 dots per inch (dpi) in color would
require 26.4 MB of data.
Also, if your printer or CRT is not capable of expressing these
differences in color and shade, there is no reason to produce such
precise (and sizable data).
8 bit 2 bit 1 bit
1-16 Overview