Thanks for the answers. Now I know that there are two ways of "SAVE AS" in Vim.
Assumed that I'm editing hello.txt.
- :w world.txt will write hello.txt's content to the file world.txt while keeping hello.txt as the opened buffer in vim.
- :sav world.txt will first write hello.txt's content to the file world.txt, then close buffer hello.txt, finally open world.txt as the current buffer.