Difference between unallocated blocks and unused blocks in database files

The blocks below the high water mark in the data file are allocated, the above are unallocated, and rows can only be inserted in allocated blocks.

For data blocks that have been allocated below the high water mark, the blocks that contain row fragments, that is, the blocks that belong to the segment, are used blocks,

A block that does not contain a line fragment, that is, does not belong to a segment, is an unused block.