These two illustrate push vs pull learning. If you want to actively assign learning content to users – build a Mission and invite users to it. If you want users to access content on a self-enrol basis – publish it in the Library.
A mission is an assignment that invited users are required to complete.
Library items sit in the Content Library, and are available to either ALL users, or to selected segments of users when/if required.
Both can be combined in several ways. While missions are great for providing must-have information within compliance training, library can offer performance support materials accessible at the point of need.