debug - Enable extra debug codepaths, like asserts and extra output. If you want to get meaningful backtraces see https://wiki.gentoo.org/wiki/Project:Quality_Assurance/Backtraces
djvu - Support DjVu, a PDF-like document format esp. suited for scanned documents
doc - Add extra documentation (API, Javadoc, etc). It is recommended to enable per package instead of globally
exif - Add support for reading EXIF headers from JPEG and TIFF images
ffmpegthumbnailer - Enable video thumbnail support with media-video/ffmpegthumbnailer
heif - Enable support for ISO/IEC 23008-12:2017 HEIF/HEIC image format