22.214.171.124.4 Density List (DList)
The Density List is a list of references to AMap pages that is sorted in order of ascending density (descending amount of free space available). Its purpose is to optimize the space allocation strategy where allocations are made from the pages with the most abundant free space first. The DList is an optional part of a PST file. However, implementations SHOULD create and use DLists.
There is at most one DList page in each PST file. If present, this page is located at absolute file offset 0x4200. To maintain backward compatibility with older clients, the location of the DList is allocated out of the Reserved data area (section 126.96.36.199) that is also used for transient storage. Because of the fact that this area is not dedicated exclusively for the DList, the DList can be over-written at any time by other transient processes and, therefore, the DList is not guaranteed to be valid. If a DList page contains an invalid CRC, then its contents MUST NOT be used and SHOULD be recreated by using the information from all of the AMap pages in the PST file. Implementations SHOULD use the DList when a valid DList exists.<15>