That's for MDC.
MDC tables maintain a block map that contains a bitmap that indicates the availability status of each block. The following attributes are coded in the bitmap list:
X (reserved): the first block contains only system information for the table.
U (in use): this block is used and associated with a dimension block index
L (loaded): this block has been loaded by a current load operation
C (check constraint): this block is set by the load operation to specify incremental constraint checking during the load.
T (refresh table): this block is set by the load operation to specify that AST maintenance is required.
F (free): If no other attribute is set, the block is considered free.
