Hello Benedict, so the main difference between MDM, EMM, and UDM is the scope of the solution. In the order from least to, the largest scope is MDM, EMM, and UEM. 

MDM focused on restricting and limiting devices to a single purpose, or task. When looking for an MDM the five main categories which are researched are Device Provisioning, Device Security, Device Health and Monitoring, Location Tracking, and Device Management.

EMM is the evolution of the MDM which basically encompasses everything an MDM perform but also provided functions like Mobile Information Management, Bring Your Own Device, Mobile Content Management, and Mobile Application Management. 

UEM is a solution to bring the management of all endpoints to a single management system from iot devices, printers, laptops, PC, printers, and wearables as well as the traditional device which were managed by MDM and EMM. 

I think a lot of it is simply marketing semantics so you may struggle to find one true definition of each. As I understand it from a marketing standpoint the difference between MDM and EMM lies more in the difference in feature sets whereas I think the distinction between a UEM and EMM is more in terms of the breadth of devices that are supported. SOTI considers itself an EMM because of all the management features and use cases that are supported and a UEM because support has been added for Linux, printers, and IoT devices in addition to standard mobile device platforms.