Jose-

Flammable =

OSHA defines "flammable liquids" as any liquids having a flash point below 100 F (37.8 C), except any mixture having components with flashpoints of 100 F (37.8 C) or higher, the total of which make up 99 percent or more of the total volume of the mixture. Flammable liquids are known as Class I liquids and are divided into three classes:
* Class IA include liquids that have flash points below 73 F. (22.8 C) and boiling points below 100 F (37.8 C)
* Class IB include liquids that have flash points below 73 F (22.8 C) and boiling points at or above 100 F (37.8 C)
* Class IC includes liquids that have flash points at or above 73 F (22.8 C) and below 100 F (37.8 C)

Combustable =

Any liquid having a flash point at or above 100 deg F (37.8 deg C) and below 200 deg F (93.3 deg C)

Or go to:

http://www.schc.org/pdf/GHS_FS_Flammable_Liquid.pdf

Good explanation.

For clarity, from Wiki: "The flash point of a volatile liquid is the lowest temperature at which it can vaporize to form an ignitable mixture in air"

Heavy fuel oil is combustible, certainly, though it needs to have its temperature raised in order to become readily flammable. The same does not apply to more volatile liquids, such as butane under pressure.