You can customize each ML differently. &fml4; and &fmldevel; are same in this point. In the case of &fml4;, there are $DIR and config.ph in ML home directory. This "config.ph" file is a perl script. There is a config.cf file in &fmldevel;, too. But the format is diffrent. The format is like this: variable = value . It is similar to postfix or .ini files. The configuration file of &fml4;, config.ph, is a perl script. Perl script is good for human editing but it is not suitable for configuration tools. It implies it is difficult to prepare configuration tools for config.ph. So, in &fml4; two files, cf and config.ph are used. It is similar to the conversion from .mc to .cf in sendmail. The use of cf is suitable for configuration tools but introduces another difficulty such as inconsistency between cf and config.ph since human may edit config.ph but forgets cf editing. So, &fmldevel; introduces a new format as .cf files. This format is postfix flavour. The format of "config.cf" is "variable = value syntax". If plural values are required, space or "\n" is a separator. variable = value valiable = value1 value2 value3 variable = value1 value2 value3 The syntax of files with the suffix .cf e.g. /etc/fml/main.cf , default_config.cf, is same. $variable at the right hand is expanded. For example, a = value1 b = $a/value2 is expanded to a = value1 b = value1/value2 . The variable expansion is done at the last. So, the following definitions a = value1 b = $a/value2/$c c = value3 a = value4 are expaned to b = valu4/value2/value3 since the last $a overwrites the previous one. variable += value variable -= value is used for adding or removing the specific value. x = a b c d x -= b becoms x = a c d . Another case, x = a b c d x += e becomes x = a b c d e . Overloading of .cf files is possible. It overwrites variables. It enables separation of configuration files e.g. the default file, the host specific file, the domain specific file, and ML specific file. Fml reads these files sequentially. At the last, fml reads ML specific config.cf and expand variables when variable reading is requested. The variable expansion is always done. If some value with $ is added to some variable, the next reading operation tie() operation of perl causes variable expansion. For example, set $config->{ key } = '$ml_home_dir/value'; here. The next read operation e.g. $config->{ another_key } (where the key is any) evaluates the variable expansion. %_fml_config hash holds pairs of key and value. The format is $dir/$file in this hash, it is not expanded. get() returns the value of %_fml_config_result. The value in this hash is after the variable expansion. The variable is expanded in calling get() method. &var.table.list.variables; &var.table.class.variables;