In SendX there are basically two ways to send email to users:
Now while sending a campaign a user may be a part of:
List is the only entity which has lifecycle events like subscribe, confirm and unsubscribe. So as a company you may have be communicating users for say:
New Product launches
To represent this you may add users to separate lists and decide to send separate campaigns to these lists (eg: Marketing Updates List and New Product Launches List )
Now users may decide to unsubscribe from only Marketing Updates or New Product launches in which case they will unsubscribe to that list only. In case user wants to unsubscribe from all communcation they will click on unsubscribe all option.
However when campaigns are sent to contacts in a tag or segment or when drip sequences are sent then the only option which users have is to unsubscribe from all communication of the company.