Where it came from
ISPM 15's publication history records the technical panel's work on sulphuryl fluoride fumigation of insects and nematodes in debarked wood before it was adopted as a treatment. It sits in Annex 1 alongside HT, DH and MB.
How it is applied
As with MB, bark must be removed before fumigation because bark can reduce how well the gas works. The schedule is a concentration-time product over a set period at a minimum wood temperature; the figures are in Annex 1 of the standard.
What the code tells you
SF in the mark means a registered provider fumigated the wood. It does not tell you anything about the pallet's structure or exchange status.
Spelling
The standard uses "sulphuryl"; US sources often write "sulfuryl". They are the same chemical.
On EPAL pallets
EPAL specifies heat treatment for its pallets. An SF code on an EPAL-branded pallet is unusual and worth a closer look at the licence markings.