HDMP, Windows Heap Dump File (.hdmp)
The HDMP System file format
HDMP files are created when a Windows program crashes or throws an error. It’s the uncompressed version of the MDMP file and contains the uncompressed data of a “dump” created when the program in question crashes. It also contains other relevant data of the system that was recorded by the time the program malfunctioned. This information can be used for debugging.
Technical details of HDMP files
Usually, HDMP files should not be opened by a Windows user. When using Microsoft Visual Studio in a controlled software environment, they can be used for debugging though. The compressed MDMP files that are created alongside the HDMP can be sent to Microsoft when reporting a bug.