No indication what "DU" means.
However, the i5-6198DU has exactly the specs as the i5-6200U except in one respect. The i5-6198DU has an Intel HD 510 graphics core rather than an Intel HD 520 graphic core. All CPUs and GPUs are mass produced and they have hundreds of millions of transistors and other parts within them. No CPU or GPU are manufactured 100% perfect, meaning every single transistor is functioning properly. That is simply impossible with current technologies. That is why CPUs and GPUs have built in redundancies and why some CPUs and GPUs can be clocked higher than others and are binned differently.
The graphics core probably had more defective transistors than normal so that the Intel HD core will not perform as well as the typical Intel HD 520 graphics core. Therefore, the defective transistor are deactivated so that the graphics cores operates like an Intel HD 510 instead. This allows Intel to "defective" CPUs at a lower price point rather than simply throwing them away and take a total loss on the CPU.
In this context, the "D" could mean DEFECTIVE.
The same applies to desktop CPUs. If all desktop quad core Skylake CPUs with no Hyper Threading capablities rolling off the production line were "perfect", they would all be i5-6600k. But that is not the case, if the CPU rolling off the production had "too many" defective transistors, then instead of being binned as an i5-6600k, it would be binned as an i5-6400 CPU... as long as it meets the minimum requirements.