UN 1993
UN 1993 — flammable liquid, n.o.s.
UN 1993 is a flammable liquid not otherwise specified. The placard names the hazard, not the exact chemical.
UN 1993 means flammable liquid, n.o.s. — not otherwise specified. The class is 3. The shipper uses this number when the material is a flammable liquid that doesn't have its own more specific identification number, or when the rules call for the generic entry. The proper shipping name on the papers fills in what the placard leaves open.
For a railfan note, "1993, class 3" is an accurate identification of the placard and an incomplete identification of the chemical. Say that plainly. Don't upgrade it to gasoline or crude because those have their own numbers and this car is not wearing them.
1993 is common on chemical tank cars for exactly this reason. It is the placard of a flammable liquid the diamond refuses to nickname.
Identification only. If a car is leaking or in a wreck, move away and call your local emergency number. Responders use the Emergency Response Guidebook. This page is not that book.