DHH is the internal memory (of sorts) for DND.
They have library racks and card catalogues of regimental histories, and scholarly studies. Those stacks are available to bone fide researchers.
DHH also receives the requisite Annual Historical Reports (AHRs). Every unit is expected to submit a formatted narrative with annexes every spring. I've done a few for two units. In one case the base was closing and I was staff at the major unit. During my walkarounds I made friends with the base graphic artist, and he gave me colour prints of many crests and semiofficial badges. I also included examples of daily paperwork and the weekly schedule. I heard back through the LCol's secretary that DHH sent back their compliments with remarks that it was one of the most extensive they'd received that year. At the other unit we put a lot of effort into remembering names and deployments, postings and promotions, and section news. The problem I recognized was the staff tended to write the narrative in the spring, and if a guy had been posted in over the summer, the AHR often missed all spring and most of the summer.
As I remember, DHH is also the repository of correct badges and regalia. So when some five watt idea takes hold to change the officers stripes to pips and crowns, except the pips are stylized maple leaves, DHH is asked to provide historical context. They also are the keepers of the colours. A unit's new colours stay at DHH between delivery from the embroiderers and consecration. In between the colours are formally just a super fancy impractical flag. Once presented, their meaning changes! Men will die protecting them. Fortunately it is a junior officer who carries the colours, and they're never going to disobey an order from a cranky old NCO with a fixed bayonet.