Usability Engineering is assumed (although not explicitly required) by the MDR in order to comply with the regulation’s general safety and performance requirements. Thus, the appropriate Usability of a medical device is an essential part needed to get clearance for the European market in the first place.
Formative usability testing is not mandatory, but it is perfect for identifying the potential for improvement of your product and for drawing up the required list of hazards and hazardous situations associated with the use of your medical device. You need this for the risk management process according to ISO 14971.
The term “usability obligation” does not exist per se. Nevertheless, Usability Engineering is indirectly mandatory for your medical device. Meanwhile, usability is perceived and required as part of product safety.