What are Microsoft Step-Up licenses and which regulations apply to their purchase?

Step-Ups are additional licenses associated to a previously acquired base license. A Step-Up license enhances the value of a base license (e.g. with a Step-Up from Office Standard to Office Professional). The following requirements need to be fulfilled:

• Step-Up licenses and base licenses need to be acquired under the same volume licensing agreement
• The base license needs to be covered by active Software Assurance
• The use of a Step-Up license requires a license for the qualifying product (base license)

Source: Microsoft Product Terms, August 2017