Data Under Errors is referring to “Weak-Bit’ Protection

Extra Tracks

Tracks 1-35 were standard. Many software houses used 36-38 as well. Anything beyond 38 was risky as some drive heads could not reliably move beyond track 38. – Jim Drew

Bad Tracks where typically “Sync-Less” Tracks.

Non-Standard format

Later on, only track 18 (the directory track) was standard! Everyone used custom disk formats to get better speed and store more data. – Jim Drew

Half Track Protection

Half tracks are something that the 1541 could not easily write without modifying the drive, or being able to get lucky and come up with a routine that would the half tracks 180 degrees out of phase (like I did with the Bounty Bob Strikes Back! copier. -Jim Drew)

Density – Frequency Protections

Changing the density level was a common trick for putting more data on a track, as well as preventing copiers from clocking in the data properly. – Jim Drew