A segment is a label that points to one or more logical devices. Use segments to improve throughput by:
Splitting large tables across disks, including tables that are partitioned for parallel query performance
Separating tables and their nonclustered indexes across disks
Separating table partitions and index across the disks
Placing the text and image page chain on a disk other than the one on which the table resides, where the pointers to the text values are stored
In addition, you can use segments to control space usage:
Tables or partitions cannot grow larger than their segment allocation. You can use segments to limit the table or partition size.
Tables or partitions on other segments cannot use the space allocated to objects on another segment.
The threshold manager monitors space usage.
