On tugs that are designed for tight constricted use where it is prudent to be able to propel both forward and aft, such as ship docking modules ( Salvador dali) and the like. The propulsion units tend to be Voith or Z drive that give equal bollard pull or push in any and each direction. Thus a tug of this calibre often finds itself entering a port facility or whatever pointing in one direction and exiting in the contra wise. Thus navigation lights to indicate the direction of travel (as one is as good as the other).
Regards
Pirate
PS this has only come to light my ways as I have and are of recent building a tug of this type/variant.