An auxillary fuel tap system meet FMVSS 301 (Federal Motor Vehicle Safety Standards), VESC-22 (Vehicle Equipment Safety Commission), NFPA 1192, ANSI 119.2, RVIA (Recreational Vehicle Industry Association) regulations, CARB (California Air Resources Board), and EPA (Environmental Protection Agency) regulations.
FMVSS 301 states that anyone who alters a vehicle must meet or exceed the structural integrity and performance of the OEM (Original Equipment Manufacturer) fuel system. A fuel system includes anything associated with fuel systems such as the fillneck, tank, gasoline lines, fuel pump, emission canister and fasteners. In other words, if an alterer adds an auxiliary system to a vehicle thereby reducing the performance of the fuel system, he or she has violated FMVSS 301 standards.
These fuel kits often are designed to retain the roll-over leak prevention of the original fuel system, which the cheaper "alternatives" don't allow for. You also have to make sure that the alterations are certified in a manner in which states that have emission testing and inspection can readily rely.
The TFI(Transfer flow) kits contain a certification decal that is placed on the driver door pillar for this reason.