The Macintosh was designed with a unique file system, featuring two forks for each file – a data fork and a resource fork. Resources stored various structured data such as dialog specifications, menus, icons, and code for apps. Developers could create new resource types using templates, and the resource editor ResEdit allowed for customization of Mac systems. However, resource forks opened up security risks as malware often exploited them. With the transition to Mac OS X, resource forks evolved into extended attributes, reducing the ability for power users to make customizations. Despite their vulnerabilities, resources and ResEdit were once essential tools for Mac users.
https://eclecticlight.co/2024/07/20/managing-classic-mac-os-resources-in-resedit/