Can pressing the back of the fingerprint with a blank signature indicate the purpose?
Of course. Don't sign on white paper easily. If you really need to sign, be sure to indicate the purpose next to the signature, so that you can indicate the purpose. Purpose, application, scope, a multi-purpose tool, but also the scope of use of various tools. Chapter 8 of Hong Qubo by Guo Moruo: "The purpose of these three funds is mainly from my idea, so I remember it clearly."