Maker notes are the manufacturer's private corner of EXIF: a single tag, MakerNote (0x927C) in the Exif IFD, typed UNDEFINED, whose contents the EXIF specification doesn't describe at all. Each vendor fills it with its own structure, often an IFD, and ExifTool decodes it from reverse-engineered tables, reporting the tag under a vendor name such as MakerNoteCanon or MakerNoteApple.
What a real maker note holds
On a Canon EOS RP JPEG the block is 15 KB and decodes into 129 tags:
$ exiftool -G1 -s -MakerNoteCanon photo.jpg
[ExifIFD] MakerNoteCanon : (Binary data 15302 bytes, use -b option to extract)
$ exiftool -G1 -s -LensType -FocusDistanceUpper -CameraTemperature -TimeZoneCity photo.jpg
[Canon] LensType : Canon EF 50mm f/1.2L USM
[Canon] FocusDistanceUpper : 10.44 m
[Canon] CameraTemperature : 34 C
[Canon] TimeZoneCity : ParisFew people expect to publish their camera's temperature or its time zone city. Several brands keep a serial number here too.
The shutter count buyers ask for on a used body also sits in this block, when the camera records one. ExifTool reads it as ShutterCount for Nikon, Pentax, many Sony models and a handful of Canon bodies (the R6 among them): exiftool -ShutterCount photo.jpg. The RP file above has none.
Why edits break them
Many maker notes contain offsets pointing at data elsewhere. Canon's count from the start of the EXIF TIFF header, so when an editor rewrites the EXIF and moves the block without fixing them, values come back as garbage. Nikon embeds its own TIFF header inside the block, which makes its offsets self-contained. ExifTool's manual calls it "a common problem with some image editors". ExifTool itself rewrites maker notes and adjusts the offsets, and its -F option tries to repair a block someone else damaged. Microsoft went its own way with an OffsetSchema tag (0xEA1D) recording how far the block moved.
The Make string matters too. ExifTool picks the Canon decoder from it, so editing Make turns all 129 tags into unknown entries.
Stripping the block
exiftool -MakerNotes:all= photo.jpgOn the RP file that removes 15,306 bytes and leaves the standard EXIF intact. ExifTool's own manual warns against running -all= on proprietary RAW files, because their maker notes often hold data the converter needs. DNG is the exception, and it even has a MakerNoteSafety tag (0xC635) saying whether the maker notes are safe to keep when the file is edited. Our reader and editor ignore maker notes; the metadata remover deletes them with everything else.