bash - What does command line arguments mean in pstree -a option

When read the manual of pstree

 -a     Show command line arguments.  If the command line of a process is swapped  out,  that  process  is
          shown in parentheses.  -a implicitly disables compaction for processes but not threads.

I am very confused about 'command line arguments'

Compare the output

me@alpha:~$ pstree |head -5
systemd-+-ModemManager---2*[{ModemManager}]
        |-NetworkManager-+-dhclient
        |                `-2*[{NetworkManager}]
        |-accounts-daemon---2*[{accounts-daemon}]
        |-acpid
me@alpha:~$ pstree -a | head -5
systemd splash
  |-ModemManager --filter-policy=strict

  |   `-2*[{ModemManager}]
  |-NetworkManager --no-daemon
  |   |-dhclient -d -q -sf /usr/lib/NetworkManager/nm-dhcp-helper -pf /run/dhclient-wlp3s0.pid -lf...

Command lines are composed of function, options and arguments

Say -a show command line arguments, implies that others just show funtions and options, but that is not the case.

What does the command line arguments mean here?

Command-line arguments, in general, refers to all arguments after the name of the program being run. For example, in your command pstree | head -5, there is one argument to head which is -5.

In the pstree output, the tree consists of a root of either a pid (process ID) or init, and then the tree of children threads. For example, in your output, NetworkManager is a parent process that is running one dhclient and 2 NetworkManager threads.

Adding the -a flag also prints the arguments that were used when each process or thread was started. For example, in your output, we can see that NetworkManager was started with one argument --no-daemon, and likewise dhclient was started with several arguments.

That is all the man page means by "show command line arguments".