To emphasise, since people seem to be getting this wrong (since "default" is overloaded): this allows you to change the default action for your conversations, i

To emphasise, since people seem to be getting this wrong (since "default" is overloaded): this allows you to change the default action for your conversations, i.e. you can explicitly set new conversations you create to automatically disappear. The default default (i.e. for new users) will still be set to keep messages.