(1) To file the deed, complete the notary form. The form will perfect the security interest.
(2a) “To file the deed, complete the notary form that will perfect the security interest.”
(2b) ”To file the deed, complete the notary form which will perfect the security interest.”
Here is what I thought. “Which,” when used as a relative pronoun, indicates that what follows was definitional and not required: “The hat, which was old, fell off.” “That” as the relative pronoun indicates a necessary further definition of the noun: “The hat that was old fell off.” Not according to Max Morenberg and Jeff Sommers, The Writer’s Options: Lessons in Style and Arrangement (Longman, 2003, 7th Ed.) at 32.
Morenberg and Sommers say that (2a) and (2b) are equally appropriate, because “form” is a thing noun. If the subject noun phrase refers to people, then you can select “who” or “that.” Thus, “To file the deed, call a lawyer, [who or that] will perfect the security interest.”