If you want to subdivide users, it is useful to create GROUPS within a list. When you import contacts, different sets of customers can be assigned to one or more groups. This will facilitate subscriber management in the future.
Instead, you will create a new LIST However, when you want to send separate newsletters or other e-mail messages to completely separate lists of customers, you will create a new LIST. A user can subscribe to more than one list at the same time: if he/she clicks on the unsubscription Unsubscribe button of one newsletter he/she will be automatically removed from that list, while remaining subscribed to all other lists.
You cannot send a message to multiple LISTS at the same time. Instead, you can certainly send a message to more than one group GROUP within the same LIST.