The Virtual Storage Map shows the size of the various common areas and the private area in address spaces. The boundaries of the common storage areas are set at IPL so the map should not change except after an IPL.
Common storage (CSA, MLPA, FLPA, PLPA, SQA and nucleus) are shared between all address spaces. Each address space has its own private area. The boundary between private and common storage is rounded to a 1MB value. Any change to the size of the private area below 16MB is particularly critical, and this report allows you to quickly see if and when there was a change.