Here’s an interesting photo I took while on my way to Kerteh:
I’ve never seen one on an open truck. We usually see these labels on bulk purpose built road kings.
If you can’t see, the truck is said to transport natrium hydroksida (sodium hydroxide or caustic acid to non-native speakers). You don’t move that stuff around in open trucks if you don’t mean to strip the flesh of passersby or spray lye along the roadway if it rains.
I guess they use the truck to move drums of the stuff around, and decided that the risk warranted the application of the sign. Very conscientious of them.
Wata - Malaysian Oil and Gas Engineer 
It’s a front so that the border patrol thinks that they are transporting chemicals, and hence don’t check, but they are really transporting something more sinister, like rice and cooking oil.
Jabba reckons they are smuggling process engineers in drums.