within a QGIS-course I would like to tell a bit about the proj.4 parameters. However, I´m not sure about the "+no_defs" parameter.

The proj.4 website (https://trac.osgeo.org/proj/wiki/GenParms) says: "+no_defs Don't use the /usr/share/proj/proj_def.dat defaults file"

Can anyone explain me what this means? (What would it mean to use the /usr/share/proj/proj_def.dat defaults file?)