Fuel tank, water tank, battery, environmental etc. PGNs works with instance field. E.g. Garmin GMI 20 can not either lock to specific source for those, but you can see different instances. Also GMI does not show anything selectable, if you do not have several instances available. But as you said one has to be carefull,but on the other hand why you would have several sensors on same tank? And if they are on different tank, they re different instances.