Next: Completion Commands, Up: Completion
A concrete example may help here. If you type M-x au
<TAB>, the <TAB> looks for alternatives (in this case,
command names) that start with `au'. There are several,
including auto-fill-mode and auto-save-mode, but they
all begin with auto-, so the `au' in the minibuffer
completes to `auto-'.
If you type <TAB> again immediately, it cannot determine the next character; it could be any of `cfilrs'. So it does not add any characters; instead, <TAB> displays a list of all possible completions in another window.
Now type f <TAB>. This <TAB> sees `auto-f'. The
only command name starting with that is auto-fill-mode, so
completion fills in the rest of that. You have been able to enter
`auto-fill-mode' by typing just au <TAB> f <TAB>.