Texinfo has the following types of @-command:
These commands start with @ followed by a letter or a word, followed by an
argument within braces. For example, the command @dfn indicates
the introductory or defining use of a term; it is used as follows: ‘In
Texinfo, @@-commands are @dfn{mark-up} commands.’
These commands occupy an entire line. The line starts with @,
followed by the name of the command (a word); for example, @center
or @cindex. If no argument is needed, the word is followed by
the end of the line. If there is an argument, it is separated from
the command name by a space. Braces are not used.
These commands are written at the start of a line, with general text on
following lines, terminated by a matching @end command on a
line of its own. For example, @example, then the lines of a
coding example, then @end example. Some of these block commands
take arguments as line commands do; for example, @enumerate A
opening an environment terminated by @end enumerate. Here
‘A’ is the argument.
These commands start with @ followed by a word followed by a
left and right- brace. These commands insert special symbols in
the document; they do not take arguments. Some examples:
@dots{} ⇒ ‘…’, @equiv{}
⇒ ‘≡’, @TeX{} ⇒ ‘TeX’, and
@bullet{} ⇒ ‘•’.
The names of commands in all of the above categories consist of
alphabetic characters, almost entirely in lower-case. Unlike those, the
non-alphabetic commands consist of an @ followed by a
punctuation mark or other character that is not part of the Latin
alphabet. Non-alphabetic commands are almost always part of text
within a paragraph. The non-alphabetic commands include @@,
@{, @}, @., @SPACE, and most of
the accent commands.
There are a handful of commands that don’t fit into any of the above
categories; for example, the obsolete command @refill, which is
always used at the end of a paragraph immediately following the final
period or other punctuation character. @refill takes no
argument and does not require braces. Likewise, @tab used in a
@multitable block does not take arguments, and is not followed
by braces.
Thus, the alphabetic commands fall into classes that have
different argument syntaxes. You cannot tell to which class a command
belongs by the appearance of its name, but you can tell by the
command’s meaning: if the command stands for a glyph, it is in
class 4 and does not require an argument; if it makes sense to use the
command among other text as part of a paragraph, the command
is in class 1 and must be followed by an argument in braces. The
non-alphabetic commands, such as @:, are exceptions to the
rule; they do not need braces.
The purpose of having different syntax for commands is to make Texinfo files easier to read, and also to help the GNU Emacs paragraph and filling commands work properly.