Yes, I believe a request for a pre approval of a multifamily building is still a purchase. I'm curious why you might think it's "not applicable"? Is there something in the following that you are questioning?
From Appendix A, A.8.
8. Request for Preapproval of a Home Purchase Loan. Indicate whether the application or loan involved a request for preapproval of a home purchase loan by entering the applicable Code from the following:
Code 1--Preapproval requested
Code 2--Preapproval not requested
Code 3--Not applicable
a. Enter Code 2 if your institution has a covered preapproval program but the applicant does not request a preapproval.
b. Enter Code 3 if your institution does not have a preapproval program as defined in Sec. 1003.2.
c. Enter Code 3 for applications or loans for home improvement or refinancing, and for purchased loans.