BCC is best used when you’re training someone and want to let them see what’s going on without being part of the action. It’s also good for keeping your supervisor in the loop.
CC should be used when you want others to know what’s going on but don’t need them to respond or take any action. Anyone who you want to be actively involved in whatever your message is about should go in the TO field. If you are asking specific things from certain people, make it very clear in body text so everyone’s on the same page. Happy emailing!