This package provides an init script and a conf file.
You don't have to modify those files directly,
but instead you can make a symlink of that init script
and a copy of that conf file.
You would do this for every server, you want to setup.

For example, you wan't to setup an world called 'gentoo',
you would do:

cd /etc/init.d
ln -s minecraft-server minecraft-server.gentoo

cd /etc/conf.d
cp minecraft-server minecraft-server.gentoo

After that, make your settings in
/etc/conf.d/minecraft-server.gentoo.

If you don't make a symlink, but use the default scripts,
your world will be called 'main'.

To interact with the console of the corresponding world,
you can use the extra command attach:

rc-service minecraft-server attach
rc-service minecraft-server.gentoo attach