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Form 637 is the federal registration that lets a company handle fuel before the per-gallon tax has been added. You need it before you can legally hold, buy, or move untaxed fuel above the rack.
Most fuel moves through the system before its federal excise tax is charged, and only registered parties are allowed to handle it that way. Form 637 is how the IRS records who those parties are. A company applies, names the activity it does, and is approved for a specific registration type, shown by a letter code.
The registration matters because the tax usually attaches when fuel crosses the rack, the loading point at a terminal where trucks fill up. To buy or sell fuel up to that point without the tax already on it, both sides generally have to be registered. Without it, you cannot operate in the untaxed part of the chain at all.
A jobber or supplier that works above the rack needs it. Suppliers will ask for your registration before they sell you fuel tax-free, and you will check a customer’s registration before you sell to them that way.
In useBefore the new supplier would sell untaxed fuel above the rack, it asked the jobber for its Form 637 registration letter to prove it was cleared to buy that way.
See also Above the rack, Motor fuel excise tax, Rack