Translations:Kernel/Gentoo Kernel Configuration Guide/17/en

Of course, for some parts of the configuration, built-in is an absolute requirement. For example, if your root partition was an  filesystem, your system would not boot if ext2 was built as a module (the system would have to look on the root partition to find the ext2 module, but it cannot look on the root partition unless it already has ext2 support loaded!).