For allpages-locked tables, the data page cluster ratio measures how well the pages are sequenced on extents, when the table is read in page-chain order. A cluster ratio of 1.0 indicates perfect sequencing. A lower cluster ratio indicates that the page chain is fragmented.
For data-only-locked tables, the data page cluster ratio measures how well the pages are packed on the extents. A cluster ratio of 1.0 indicates complete packing of extents. A low data page cluster ratio indicates that extents allocated to the table contain empty pages.
For an example of how the data page cluster ratio is used, see “How cluster ratios affect large I/O estimates”.