Formats

CULL shows you the preview your camera saved inside each RAW file. Most cameras save one at full size, so zooming in shows every pixel. Some save a smaller one; then the app zooms as far as that preview goes and says so in a small badge.

Canon

CR3 and CR2. Full-size previews on every camera.

Nikon

NEF. Full-size previews on every camera.

Sony

ARW. Cameras from 2021 on save a full-size preview; older ones a smaller one.

Fujifilm

RAF. Every camera saves a preview at about half size.

Olympus and OM System

ORF. Every camera saves a 3200-pixel preview.

Panasonic and Leica

RW2 and RWL. Cameras from 2022 on save a full-size preview; older ones a smaller one.

DNG

Whatever wrote the file decides. Cameras that shoot DNG save full size; converters and phones usually save a small one.

What is written, and where

A keep is Lightroom's pick flag, a reject its reject flag, and a favorite a pick with a purple label. They go into an XMP sidecar next to the photo. A DNG carries its own metadata and Lightroom ignores a sidecar beside one, so there the verdict goes inside the file. The image data is never changed.

Not supported

Pentax PEF, Samsung SRW, Hasselblad 3FR, Sigma X3F and the old Canon CRW. Most of those cameras can also shoot DNG.