ICC profile.
The one metadata block most people should keep — and the one that shares a marker with the block most likely to leak a second photograph.
An ICC profile describes the colour space an image's numbers are expressed in, so a display can reproduce the colours the camera intended. In a JPEG it is carried in one or more APP2 segments identified by ICC_PROFILE\0.
What it is
Without a profile, a viewer must guess a colour space — usually sRGB. If the file is actually Display P3 or Adobe RGB, that guess is wrong and the colours shift, most visibly in saturated reds and greens. Profiles are frequently larger than a JPEG segment allows, so they are split across several APP2 segments and reassembled.
Why it matters
Dropping the profile changes what the picture looks like. That makes it categorically different from EXIF or XMP, which change nothing visible at all, and it is why ScrubPony keeps it by default.
It is not perfectly innocent: an unusual profile does narrow down the camera or software that produced a file. That is a real, if weak, signal, so --strict drops it — at the cost of colour fidelity, deliberately chosen rather than defaulted into.
--strict it is dropped, with the reason uncommon profiles narrow the source. It shares APP2 with MPF, which is always dropped — the same marker, opposite fates, decided entirely by the identifier.
Related terms
Common questions.
Will my photos look wrong if I use --strict?
On a wide-gamut display, quite possibly — colours will look flatter or differently saturated. On a plain sRGB screen with an sRGB image, you will not notice.
Does keeping the profile leak anything?
Weakly. A standard sRGB profile says nothing. A vendor-specific profile narrows down the device or software. Judge the file, then choose the flag.
Get ScrubPony
Strips identifying metadata out of JPEGs without touching a pixel. C99, no dependencies beyond libc, macOS and Linux, MIT-licensed.