The "A" designator means the hose can be used in enclosed spaces, i.e., it passed a 2.5-minute fire test;
The "B" designator means the hose should be used only in open, above deck spaces, i.e., it has not passed a 2.5-minute fire test.
The numerical suffix indicates the suitability for certain applications:
--the "1" suffix indicates the hose is designed to have fuel in the hose all the time, i.e., it has low (less) permeability;
--the "2" suffix indicates the hose is NOT designed to have fuel in the hose all the time, e.g., used for a tank filler hose; i.e., it has higher (more) permeability;
For an authoritative reference, see
For an on-deck, open fuel system, the hose from the fuel tank to the engine can be rated B1, but A1 is better and can be used as well. (On my boat, all fuel lines are rated A1 except the below-deck fuel tank filler hose and the tank vent line.)
All hoses should be clearly marked in plain text as being ALCOHOL RESISTANT. Generally you only find non-alcohol-resistant fuel lines in older boats, made before c.1987.
There is a great deal of field experience with failure of fuel hoses with ethanol-gasoline blended fuels, particularly with fuel hoses having a gray metallic outer jacket. These were common in Mercury installations and were sold by several aftermarket suppliers.
A second numerical suffix of "15" refers to a new rating for ultra-low permeation. This is now required by EPA regulations, and is not a Coast Guard rating.
The USCG safety ratings do not meet the EPA requirements for fuel line permeation, which shall
be less than 15g/m^2/day when tested according to SAE J1527 FEB 2011. EPA-compliant fuel
hose is labeled A1-15 or B1-15 showing the A or B fire rating and the permeation at 15 g/m/day.