A bulk discount occurs when the number of total number of uses, regardless of which piece of content is used, within either a Project or throughout a License reaches a specified quantity.
A multi-use discount occurs when the same piece of content is used more than once within the same project. The discount specifies how many times the use of a piece of content must occur within a project before receiving a multi-use discount and which of the uses receives the discount.
A relicense discount is one that occurs when a project spawns a derivative work, perhaps the second edition of a book or alternate language for a website, and a use is contained in both original and derivative Projects. Only the use from the derivative Project would receive the discount.