the config file specifies if something is compiled into the kernel (y) or as a module (m)or not at all (commented out). Example, for MD related stuff (from an older build enviroment on my computer):
Often, the config file is auto generated from somewhere else. It has been a few months since I have built the kernel, so I forget some details.
doug@s15:~/temp-18.104.22.168/linux-3.2.0$ grep "CONFIG_MD" config