Disc Record explained:

Offset Meaning

3       Density
        1       Single Density
        2       Double density
        4       Quad Density
        This is independant of bytes per sector.

A Disc Record may get updated in parts at different times when a disc is being mounted:
Mount (fields which are filled in by ADFS/SCSIFS/other when the disc is Mounted by miscellaneous reason code Mount): SectorSize
SecsPerTrk
Heads
Density
LowSector

StartUp (fields which are filled in by FileCore after the disc has been identified as a FileCore disc using ServiceCall_IdentifyDisc): LinkBits
BitSize
RASkew
BootOpt
Zones
ZoneSpare
RootDir

Identify (fields which are filled in by, or modified by FileCore, or whatever MultiFS filing system which has identified the disc):

  Heads                 (when Sequence sides is altered, Heads must be
                        altered in step with it)

When FileCore tries to match a mounted, positively identified disc disc against those which it has already seen it bases its comparison on these fields:
SectorSize
SecsPerTrk
Heads
LowSector
SequenceSides
DoubleStep
DiscId
DiscName
These are the fields which get filled in by Mount and Identify.