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
  -V, --version  Print version

$ 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

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.