Common practice is an amazingly local thing. Typically a plant or facility will refer to product in the same terms as the sales contract at that facility. Many localities sell liquids by weight and use mass terminology, others use volume terms and correct to a specified temperature. I see bulk oil in mt pretty often associated with ship loading/unloading facilities.
I work all over the world and it always amazes me when I meet someone my age who has only worked at one facility for an entire career. These are the guys that truly believe that the way things are done within that one plant were ordained by a higher power and everyone else either does it the same way or is wrong. If the facility sells liquids by tonne, then they will argue that there is no way to get accurate volume measurement without knowing the weight and density. If the facility sells liquids by the m^3 then they will contend that inherent errors in weight instruments make volume the only valid way to assess a quantity in liquids. They are both wrong, but will not be convinced that there is validity to another method. It makes the life of a consultant interesting.
The response from Compositepro (that "mt" = metric ton) is probably correct. However, be aware that "Mt" is megatonne or 10<sup>6</sup>metric ton.
Also, be aware that many people in metric nations would say that "t" (without the "m" in front of it) is a "tonne" which is a metric ton (i.e., 1000 kilograms).
See this article in the online encyclopedia Citizendium for even more information on this subject:
Milton,
Looking at the picture of the tag my guess is that it is old enough that the people putting it up wanted to make sure that it was "clear" that the tank was measured in weight and that the weight was "tonne" not "ton" (8 years ago "mt" was more common than "tonne" and there was a lot of confusion when people suddenly changed nomenclature).
mt=metric ton
a tank of 1mt can hold 1000 liter of water
1mt = volume water 1m x 1m x 1m = 1 m3
the specific gravity is considered to be 1
so 1mt=1000kg or 1ton
metric system=easy?
by defining a tank in mt, one doesn't need to specify
specific gravity liquid
The tank need to have both the volume and straight to contain that amount of water (sg=1)
water and hydrogen are often used as reference=1
mendeljev:hydrogen has 1 electron
hydrogen molar mass (1)
water specific gravity (1)
...
