What does it mean if the TTY field is a question mark?

Question

What does it mean when ps lists a process's TTY as ? instead of a tty device name/number?

For example:

piotr@thinkpad:~$ ps -A | head -2 
    PID TTY          TIME CMD
      1 ?        00:00:04 systemd

The TTY is represented by ? and for example bash TTY is 0

Answer 1

It has disassociated itself from the controlling pty.

See:

• Why MUST detach from tty when writing a linux daemon?