The looped carriage tape moves synchronously with the stream of fanfold paper.
The line printers have 12 sensors to recognize 12 independent positions on the carriage control tape.
By convention the position on a sheet of paper where the first print line has to be written is associated with Channel 1.
By convention, IBM mainframe applications always jump to channel 1 when beginning a new logical page.
Print data sets on IBM mainframe operating systems may have either of two variants of printer control characters: The attribute for specifying the presence of print control characters is part of the Record Format (aka RECFM) attribute must therefore allow for two variants: ASA control characters are logical printer commands.
Machine control characters, in contrast, are the hardware commands which IBM line printers understand.
Many programming languages simply place the desired control character in the first byte of the line to be printed.
The first set of commands did not send any data to be printed to the printer but only a paper movement instruction.
The second set of commands send data to be printed on the current line plus a paper movement instruction to the printer.