A co-applicant and co-borrower are the same thing, just at different points in the loan process; once a loan is funded, a co-applicant becomes a co-borrower. In this case, both the primary applicant and co-applicant sign the loan agreement and are jointly liable for the loan. A co-applicant also must live at the same address as the primary applicant.
On the other hand, a co-signer on a student loan or mortgage is jointly liable but does not have the same interest in the loan and does not sign the loan agreement (only the co-signer addendum). SoFi does not offer co-signers on Personal Loans.