Maker notes.
The manufacturer's private appendix to your photograph. Undocumented, unstandardised, and exactly where a serial number goes.
Maker notes are vendor-specific metadata written by camera and phone manufacturers. Some sit in a dedicated EXIF tag; others occupy whole APPn segments with proprietary formats and no published specification.
What it is
Every manufacturer records more than the standard has room for: lens serial numbers, body serial numbers, shutter counts, firmware versions, autofocus point selections, internal temperature. The formats are proprietary, frequently undocumented, and change between models.
Why it matters
A body serial number in a maker note links every photograph from that camera to every other one, across accounts and platforms and years. It is a stable identifier that most people do not know they are publishing.
Because the formats are undocumented, a scrubber cannot reason about the contents of an unfamiliar vendor block. That leaves two options: keep it and hope, or drop it. Keeping it is how tools quietly leak.
APPn are classified as unrecognised and dropped by default, with the reason unrecognised application segment. That default is the fail-closed rule: an undocumented segment is exactly where a manufacturer parks a serial number, and a privacy tool that fails open is not a privacy tool.
Related terms
Common questions.
Could dropping an unknown segment break a file?
In principle. In practice, decoders ignore application segments they do not recognise, which is what the marker range is for. If it ever does break something, that is a bug report worth receiving and a new row in the policy table — not a reason to invert the default.
Can I keep a specific vendor segment?
No. There is deliberately no policy override: the fastest route back to leaking GPS is a configuration file somebody edited once and forgot.
Get ScrubPony
Strips identifying metadata out of JPEGs without touching a pixel. C99, no dependencies beyond libc, macOS and Linux, MIT-licensed.