DEMUs were significantly different to all other diesel multiple unist when the TOPS scheme was first mapped out:
1) they (generally) had bogies that were connected to the prime mover by power cables, i.e. no cardan shafts
2) as a result of 1) above their maintenance regime was considerably different, involving high power traction electrics, - all other DMUs only had moderate electrical power arrangements for auxiliaries and hotel supplies
3) potentially, DEMU interoperability with S & T has always required more attention to detail above the normal issues of axle loading and gauging
In these more enlightened times, there is now even more reason to demark DEMUs from DMUs fitted with either mechanical of hydraulic transmissions as unlike the others, DEMUs are not dead-end technology, - forever relying on burning hydrocarbons. DEMUs have more in common with EMUs and BEMUs in terms of their driving characteristics and by way of bi-mode capabilities, bring the opportunity of reduced or even zero CO2 and pollution in sensitive areas.