HP PCL/PJL reference (PCL 5 Printer Language) - Technical Reference Manual Part II

EN Enhancing Labels 23-11
These lines use one (X,Y) coordinate pair, one LO command to
center labels, and a Carriage Return and Line Feed after each
line. Without this command, an alternative method would involve
calculating the length of the line in CP (Character Plot) cells, dividing
by two, and using the CP command to ‘Backspace’ the required
number of cells. The LO command saves calculation, decreases
the number of characters sent to the printer, and allows you to take
advantage of proportional fonts when the character widths are not
known to the software.
Terminating Labels
LB tells the printer to print every character following the command,
rather than interpreting the characters as graphics commands. In
order to allow the normal terminator, the semicolon (;), to be used in
text, the command is defined so that you must use the special ‘print
label terminator mode’ to tell the printer to once again interpret
characters as graphics commands. (If the command had been
defined otherwise, you wouldn’t be able to print semicolons in your
text.)
The default label terminator is the non-printing ASCII end-of-text
character ETX (decimal code 3). You must use the label terminator,
or the printer prints the rest of your file as text instead of executing
the commands. You can change the label terminator using the Define
Label Terminator (DT) command.