You can customize each ML differently. fml4 and fml8 are same in this point.
In the case of fml4, there is a config.ph in ML home directory, $DIR (e.g. /var/spool/ml). This "config.ph" file is a perl script.
There is one configuration file (config.cf) in fml8, too. But the format is different. The format is like this:
variable = value. It is similar to postfix or .ini files.
It is similar to a perl module. You can write perl codes after =cut line.
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. 
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 editing cf. So, fml8 introduces only one configuration file. It is a new format which is similar to Postfix main.cf.
It is similar to the conversion from .mc to .cf in sendmail.
Copyright (C) 1993-2018 Ken'ichi Fukamachi mail:< fukachan at fml.org >