Sequential scans include table scans, clustered index scans, and covered nonclustered index scans.
During table scans and clustered index scans, asynchronous prefetch uses allocation page information about the pages used by the object to construct the look-ahead set. Each time a page is fetched from a new allocation unit, the look-ahead set is built from all the pages on that allocation unit that are used by the object.
The number of times a sequential scan hops between allocation units is kept to measure fragmentation of the page chain. This value is used to adapt the size of the look-ahead set so that large numbers of pages are prefetched when fragmentation is low, and smaller numbers of pages are fetched when fragmentation is high. For more information, see “Page chain fragmentation”.