(Had this on file for a while, so I thought I'd share it. Can't think of any new roles to define for the Agitator universe at the moment...)
When it was introduced the KR-14 was supposed to be the end-all standard global Axel used by the nations under the flag of the UEPC. Of course, now that the UEPC has control over all world governments, this is the case.
But it became clear over the years that the quality of the KR-14 was sub-par to some other designs produced by pirate companies (as most private Axel manufactures were snuffed out by heavy government regulation of the economy). In order to dissuade countries and organizations from encouraging sales of these pirate Axels, the UEPC publicly announced a plan to upgrade the KR-14 model and address several issues. The result was dismissed by some as a 'half-assed' attempt at a solution, and the KR-14 is just as expendable as it was in the A model.
Despite being as lackluster as its predecessor, the D model does address some concerns. Foremost of all was the articulation of the arms. The stiff 'shoulder' parts connecting the arm blocks to the body was revisited to give the unit more mobility there (articulation on-par with the legs). This came at the cost of a cramped cockpit, but most pilots found the articulation welcome when in tight urban spots.
Another major point addressed was the area in the back of the knee. Most Axels use calf-guards for balance purposes, but when the KR-14A model did not it was seen as a triumph in balance technologies. However, the flaw was exploited in a number of regions by guerrilla soldiers with anti-tank rockets, and most KR-14s could be immobilized from a direct hit. Despite keeping the new balance system, the calf-guards were added anyway and claimed to be part of 're-armoring' the unit.
The third modification was re-tooling the arm's balance systems, trading out the stacked systems with single units. While this was done in the name of "performance", it is largely seen as cutting costs and removing the redundancy of the stacked systems. As a result, rare errors of balance inconstancy between the arm and leg monitors caused the unit to topple when the gyros compensated. The third run of the D model appears to have corrected this, but quality control remains a problem (and not much of a concern, given how little the earth-bond units actually see combat).
Other problems of the KR-14 model remain, such as the black-and-white cockpit monitors, standard armor and tricky articulation across the waist.
Most of the D models are deployed on Earth, to the units closest to the factories. New units are being sent into space, but the UEPC doesn't appear to be in a hurry to phase out the A model, which is the most prevalent axel in UEPC controlled territory.