Its a rough guide. And its US gallons yes. For instance lets say one had a 10 gallon tank according to the rule one could put a 10 inch cichlid in the tank. So use common sense.
Fish with longer slender bodies need longer tanks while fish with fuller deeper bodies like gold fish, angels, discus need tall tanks. A 100 gallons may be enough for a discus but if its only a foot high theyll struggle.
Other fish like live bearers, plecos, goldfish create more waste than other fish and as such need more gallons to compensate for. Its all about available oxygen in the water. In a 55 gallon one could have over 55 1 inch tetras as they create little waste and dont require as much oxygen as say a goldfish where as in that same tank would be suitable for 2 or 3 golfish even though 3 gold fish would be well under 55 inches of fish.
Also warmer water contains less oxygen than cold water so the warmer the water like 85 degrees for discus the less available oxygen and less room for fish.
Do your research before stocking make sure fish are GH compatible, temperment compatible, temperature compatible and your fish will be less stressed as will you.
Good luck ask questions about stocking and there are plenty people here that will give suggestions.
