Under 1026.17(d), In transactions that are not rescindable, the initial CD required by § 1026.19(f) may be provided to any consumer with primary liability on the obligation.

What is meant by primary liability? I was unable to locate a definition.

It basically means any joint co-borrower as all co-borrowers would be primary liable for the debt, of which a guarantor or co-signer would not be consider primary liable.