If this is set to 'true', all parameters/variables in the main template will be available in the nested vstemplates with ...

If this is set to 'true', all parameters/variables in the main template will be available in the nested vstemplates with an 'ext_' prefix. For example the main template parameter $safeprojectname$ will become $ext_safeprojectname$ in the nested template - the nested template will therefore have $safeprojectname$ and $ext_safeprojectname$.