What characteristics make a good clustered index key?

A useful clustered key is commonly narrow, stable, unique or made unique internally, and chosen to support important access patterns. An ever-increasing key can reduce random page splits for many insert-heavy OLTP workloads.

Because the clustered key is carried as the row locator in nonclustered indexes, a very wide clustered key increases the size of those indexes as well.

There is no universal rule that every clustered key must be an IDENTITY integer. The correct choice depends on query patterns, insert behavior, uniqueness, storage cost, and data lifecycle.