When sending an email, the action you want from the recipient(s) dictates which address box you put their name.
‘To’ is for action. You expect an action from those whose name is in that line. If there are multiple people in the To box be sure to spell out who is expected to take action (when and how).
‘Cc’ is for information only. It implies that no reply is either expected or necessary. Good email etiquette dictates that if you are in the Cc line and feel you must voice your opinion, only reply to the sender.