its the name of VAG/Febi brand coolant. G11 is older VAG stuff, replaced by G12 which wont mix with G11, cue brown sludge. then VAG replaced G12 with G12+, which mixes with both G11 and G12, so no more sludge issues. I think they have also now brought out G12++, no idea what the difference with that stuff is. I just get whatever GSF/VW or AVS sell, depending on who I'm ordering from at the time.