macports.conf vs macports.conf.default

[M A]

Under /opt/local/etc/macports I have both a macports.conf and
macports.conf.default file. The first seems to be from 2010 and the
latter from today. I guess the port command should be reading the
newer, but the documentation on the web (and using 'man port' and 'man
macports.conf') says that macports.conf is used. Is this correct? And,
if so should I replace macport.conf with macports.conf.default?

The primary reason I'm asking is because I want to make some changes
and put that into ~/.macports/macports.conf. Can I put in only the
changes, or is it necessary to copy the whole macports.conf file from
/opt/local/etc/macports?

Thanks,
Mark A