.so filenamereads input from filename, then continues with the current file.
.nx filenamecontinues with filename and doesn't come back. (Like a "goto".)
reading standard input
.rd prompt arg1 arg2 ...prompts with prompt: (default: beep, if input is from a terminal), and copies text from standard input until an empty line. Arguments to .rd after the prompt can be accessed as \$1, \$2, ...
\!textwrites text directly to the formatter. (E.g., PostScript images to a printer that understands them.)
.tm messageprints the message to the standard error stream, presumably the user's terminal.
.flflushes output. (E.g., for debugging.)
piping nroff output to commands
.pi commandpipe output to command (nroff only). This must occur before any output. Or before the first BUFSIZ bytes of output, presumably.
Only the first word in command is used, and it's opened without evaluating $PATH; use absolute pathnames or die.
.exends processing. (The manual has that as .ex prompt, but I assume that prompt was carried over from the previous item, rd.)
.ab textprints text and aborts.