The SQA/CSA Usage report shows the used and free space in the SQA and CSA areas of virtual storage.
SQA and CSA are areas of common storage i.e. they are shared by all address spaces. The size of the areas is fixed at IPL but the space used can vary over time.
If all SQA space is used the SQA can expand into CSA. This is not necessarily a problem - it is only a problem if all CSA is also used.