While editing, you will sometimes want to save just part of your file as a separate, new file. For example, you might have entered formatting codes and text that you want to use as a header for several files.
You can combine ex line addressing (Section 20.3) with the write command,
w, to save part of a file. For example,
if you are in the file practice and want to save part of
practice as the file newfile, you could enter:
:230,$w
newfileSaves from line 230 to end-of-file in newfile.
:.,600w
newfileSaves from the current line to line 600 in newfile.
After newfile has been created, you'll need w! instead of w.
— LL, from Learning the vi Editor (O'Reilly, 1998)