In this section, I explain what each of the DBCC CHECK. . . commands does. Historically, there has been much confusion about what all the different consistency-checking DBCC commands do, which ones should be performed, and in what order. DBCC CHECKDB includes the functionality of all DBCC CHECK. . . commands except DBCC CHECKIDENT and DBCC CHECKCONSTRAINTS.
DBCC CHECKALLOC performs the following:
Primitive system-catalog consistency checks
Allocation consistency checks on the database
It uses a database snapshot by default and has the same options as for DBCC CHECKDB, except for the following:
None of these options make sense for allocation consistency ...