These are simply abbreviations for your van's dimensions.
L refers to your van's length or wheelbase.
H refers to your van's roof height.
As a general rule the smaller the number the smaller the size.

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L1 = Short Wheelbase (SWB)
L2 = Medium Wheelbase (MWB)
L3 = Long Wheelbase (LWB)
H1 = Low Roof
H2 = Medium Roof
H3 = High Roof

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If you are not sure what your van's wheelbase or roof height is, please contact our customer team who will be happy to assist. Products without an L or H value assigned are universal fitting and should fit most vans.