Reallocated sectors (5) are sectors the drive has retired and replaced with spares after they failed to read reliably; a handful is normal, a rising count is a surface going. Current pending sectors (197) are sectors that failed a read and are waiting to be reallocated on the next write; they are the important number, because every one of them is a sector the drive cannot currently give, and a rebuild will ask for all of them. Offline uncorrectable (198) are pending sectors the drive's own scan confirmed. Reported uncorrectable (187) and command timeout (188) are the errors and stalls the host saw, which is what a controller acts on. CRC errors (199) are almost always a cable.
A SAS drive says the same things differently. The grown defect list is the list of sectors it has reallocated since manufacture, and the controller reads its length; the read and write error counter pages give the corrected and uncorrected totals; and a failed read comes back with a sense key that says why: medium error for a sector it could not read, hardware error for a mechanical or electronic fault, not ready for a drive that has not come up, with additional codes such as 04/00 for not ready with no cause reportable, 04/04 for format in progress, and 03/31/00 for medium format corrupted, which is what Dell's LT-series SSDs reported after their 40,000-hour failure.
The controller's threshold is a guess about how long the drive has left, and on a healthy set it is a good guess: replace the drive, let the rebuild read the clean survivors, and carry on. On a degraded set the same replacement asks the rebuild to read the predictive drive from end to end, including its pending sectors, and the first one it hits is where the rebuild stops. Imaging the drive first reads those sectors the slow way, with retries the controller would never allow, and hands back a copy that answers every one. That is the difference between a replacement and a recovery, and the numbers say which you need.