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For certain kinds of arguments, you can use completion to enter the argument value. Completion means that you type part of the argument, then Emacs visibly fills in the rest, or as much as can be determined from the part you have typed.
When completion is available, certain keys---
SPC---are rebound to complete the text present in the minibuffer
into a longer string that it stands for, by matching it against a set of
completion alternatives provided by the command reading the
argument. ? is defined to display a list of possible completions
of what you have inserted.
For example, when M-x uses the minibuffer to read the name of a
command, it provides a list of all available Emacs command names to
complete against. The completion keys match the text in the minibuffer
against all the command names, find any additional name characters
implied by the ones already present in the minibuffer, and add those
characters to the ones you have given. This is what makes it possible
to type M-x ins
RET instead of M-x insert-buffer
RET (for example).
Case is normally significant in completion, because it is significant
in most of the names that you can complete (buffer names, file names and
command names). Thus, `
fo' does not complete to `
Completion does ignore case distinctions for certain arguments in which
case does not matter.