nl considers its input to be composed of logical pages; by default, the line number is reset to 1 at the top of each logical page. nl treats all of the input files as a single document; it does not reset line numbers or logical pages between files.
A logical page consists of three sections: header, body, and footer. Any of the sections can be empty. Each can be numbered in a different style from the others.
The beginnings of the sections of logical pages are indicated in the input file by a line containing nothing except one of the delimiter strings shown below:
\:\:\: start of header \:\: start of body \: start of footer
The two characters from which these strings are made can be changed with an option (see below), but the pattern and length of each string cannot be changed.
The section delimiter strings are replaced by an empty line on output. Any text that comes before the first section delimiter string in the input file is considered to be part of a body section, so a file that does not contain any section delimiter strings is considered to consist of a single body section.