On the exemption certificate, the CRA will likely publish a model with the information that is required. It is a piece of paper that the farmer is going to fill out. Essentially it works for every delivery of fuel the farmer will receive. He will have to present the certificate.
You can foresee that in an ongoing relationship with a fuel supplier, the exemption certificate may be on record at the fuel distributor. Basically he's just asking whether the farmer will use the exemption certificate when he wants delivery of a particular fuel.