pacman defines subcommands via flags.
Here, -S is a short flag subcommand:
$ pacman -S package
Installing package...
Here --sync is a long flag subcommand:
$ pacman --sync package
Installing package...
Now the short flag subcommand (-S) with a long flag:
$ pacman -S --search name
Searching for name...
And the various forms of short flags that work:
$ pacman -S -s name
Searching for name...
$ pacman -Ss name
Searching for name...
(users can "stack" short subcommands with short flags or with other short flag subcommands)
In the help, this looks like:
$ pacman -h
package manager utility
Usage: pacman[EXE] <COMMAND>
Commands:
query, -Q, --query Query the package database.
sync, -S, --sync Synchronize packages.
help Print this message or the help of the given subcommand(s)
Options:
-h, --help Print help information
-V, --version Print version information
$ pacman -S -h
Synchronize packages.
Usage: pacman[EXE] {sync|--sync|-S} [OPTIONS] [package]...
Arguments:
[package]... packages
Options:
-s, --search <search>... search remote repositories for matching strings
-i, --info view package information
-h, --help Print help information
And errors:
$ pacman -S -s foo -i bar
? failed
error: The argument '--search <search>...' cannot be used with '--info'
Usage: pacman[EXE] {sync|--sync|-S} --search <search>... <package>...
For more information try '--help'
NOTE: Keep in mind that subcommands, flags, and long flags are case sensitive: -Q and -q are different flags/subcommands. For example, you can have both -Q subcommand and -q flag, and they will be properly disambiguated.
Let's make a quick program to illustrate.