Count of reallocated sectors. When the hard drive finds a read/write/verification error, it marks this sector as “reallocated” and transfers data to a special reserved area (spare area).
This process is also known as remapping and “reallocated” sectors are called remaps. This is why, on a modern hard disks, you can not see “bad blocks” while testing the surface – all bad blocks are hidden in reallocated sectors. However, the more sectors that are reallocated, the more a sudden decrease (up to 10% and more) can be noticed in the disk read/write speed.