These checks verify the contents of and relationships between the various structures that track page and extent allocations in the database. The structures involved are as follows:
PFS pages, which track the allocation status of individual pages within a 64-MB section of a data file (a PFS interval)
GAM pages, which track the allocation status of all extents within a 4-GB section of a data file (a GAM interval)
SGAM pages, which track all mixed extents that have at least one page available for allocation with a GAM interval
IAM pages, which track all pages and extents allocated to an allocation unit from a GAM interval
IAM chains, which are a linked-list of all IAM pages for an allocation unit (and hence track all pages ...