In France, a refund claim from a company outside the EU must be filed by a representative accredited by the tax authority. VATBAK files for you, through MS NISSA.
A 13th Directive claim in France is filed by a representative accredited by the DGFiP, never by the claimant. VATBAK claims are filed by MS NISSA, accredited under article 289 A of the CGI and acting as its clients' representative. That is what lets VATBAK handle this leg at all: most recovery platforms cannot.