https://android.googlesource.com/platform/packages/modules/adb/+/d0db47dcdf941673f405e1095e6ffb5e565902e5/adb.h#199 https://android.googlesource.com/platform/system/core/+/7199051aaf0ddfa2849650933119307327d8669c/fastboot/fastboot.cpp#244
Files in this directory contain configuration for systemd-udevd.service, a daemon that manages symlinks to device nodes, permissions of devices nodes, emits device events for userspace, and renames network interfaces. See man:udev(7) for an overview of the configuration file format, and man:systemd-udevd.service(8) for a description of service itself. Use 'systemd-analyze cat-config udev/rules.d' to display the effective config.