MrFlibble:
I’d assumed (by reading between the lines of your previous answers) that if it were permanently fixed to the chassis, that made it a tank. Perhaps because it looks like it’s bolted down (and therefore technically removable if you undo the bolts?) I guess I don’t really need to know what defines the difference between a tank and a package, but curiosity gets the better of meThanks for the help
You’re correct again Mr Flibble. 
The bolts are simply a convenient way of securing the IBC (package) to the trailer.
They are easily removed and the IBC is then able to be handled mechanically (a defining requirement for IBCs) so as to be ADR compliant.
The difference is in the word “permanently” when describing the attachment.
This might sound a little confusing, but since you’re curiosity has been aroused…
(The following is regardless of the vehicle’s own fuel tank for which the limit is 1,500L.)
When a (cargo carrying) tank is permanently fixed to a vehicle, the tank is then counted as a “fixed tank.”
The vehicle to which a fixed tank is attached, then becomes a “tank vehicle” (= tanker)
When a tank vehicle contains any amount of any dangerous goods, the driver needs a full ADR licence valid for tanks and the appropriate UN Class of dangerous goods carried once the carrying tank exceeds 1,000L capacity.
I’ve had a look at the item you suggested in the side-bar of the link, and although I can see “ADR approved” etc, I noticed that I’m not seeing any evidence of the approval.
(That doesn’t mean that I suspect that it’s not approved.)
I would count this as evidence of approval:
You can clearly see this in both pics I posted above.
This next pic is also of an IBC bowser including the plate above, but this one is 2,000L size, so the driver would need an ADR licence valid for packages and UN Class 3. This one carries Jet A-1, which is slightly more flammable than Diesel but still with a 1,000L ADR limit.
Some IBCs can be up to 3,000L.
Click on pic to see full size
As an aside, I’m currently dealing with exactly this issue (approval) for another customer as we write. Spooky! 