Citizen CLP-6401 User Manual page 34

Reference line and points
The reference line and points are described here. The position of 2.5
mm from the left edge of the paper is the reference line for paper.
Always the left bottom is the reference point for printing characters and
bar codes. The concept of this reference line and points are common to
such commands as ruled line and graphics.
Printing reference point
Printing reference point
2.5
Left edge of paper
Paper reference line
Printing reference point
Printing reference point
Direction of paper feed
3-5