RFE response is via fax or postal service?

My H1B transfer petition filed in premium processing on 6/9 got an RFE on 6/16 for Specialty Occupation, Employer/Employee Relationship and Client Letter. My employer confirmed that they are sending the RFE response yesterday on 6/30.

I want to know this RFE response documents are sent to USCIS via fax or hard copies via post?