Tell them that I'm undocumented and paying with cash? (I'm not undocumented, but I imagine they see that enough which is why they don't bother saying anything

Tell them that I'm undocumented and paying with cash? (I'm not undocumented, but I imagine they see that enough which is why they don't bother saying anything when I leave it blank.) I'm pretty sure they just want your SSN for collections/credit-reporting purposes.