I do not know what this is, but some copy protection software mentioned this in their ad.
Anyone have information on this one?

Sync Bytes Copy Protection

Sometimes you would find protections counting the number of sync bits in a row or timing the duration of series of sync bytes to see if they matched. These were difficult to duplicate because we really didn’t have much control of the data separator. We had to write routines that calculated the bit length for every sequence and handle sync bits a little differently. – Jim Drew

Checksums – Copy Protections
Typically error 23 was used. Later on, when the job queue was used for reading errors without rattling the head, people figured out you could get valid data even though the checksum was wrong. – Jim Drew

GAPS – Copy Protections

The space between sectors or between the track end and start. Gaps were not used very often in protection schemes, especially when hardware copiers came out. – Jim Drew

Track and Sector ID’s Copy Protection

It was not uncommon in early protection schemes to have two or more identical sectors on the same track, or even multiple tracks. -Jim Drew

Drive Errors

Typical errors 21,22,23,27,and 29. These were all typical error codes used in the early copy protection.

These are “Read Errors”
#20 – Block Header was not found
#21 – No Synch Character
#22 – Data Block not found
#23 – Checksum Error in Data Block
#27 – Checksum Error in Header
#30 – General Syntax Error
For reference, #24 is also a read error, #25 is a write error, #26 is write protection is on, #29 is Disk ID Mismatch.

This protection worked by putting a certain number of errors on a disk.
The program would then check/count the number of error messages and would not load if the number was not correct.