Special marker used to separate different areas of data.
The sync is (in theory) not produced by the Encoding used on a data area. Therefore it can mark the beginning (sometimes not really the beginning) of a new physical/logical data area, known as a Block (aka Sector). The PC and the Commodore Amiga’s “AmigaDOS” format both use MFM (Modified Frequency Modulation) “0×4489” as the sync pattern, however it is possible for custom Amiga formats to have many different sync values on the very same Track.
In the analyser it is referred to as the “Mark” to be more in line with Trace conventions. It is treated as data, but the encoding can be modified, but it is used by the analyser to spot areas on a track in the first place.