What do you call someone who signs a document on behalf of another?

The term for someone who signs a document on behalf of another person is "Attorney in Fact." This individual has been granted the authority to act on behalf of another, usually through a legal document known as a power of attorney. This role can involve signing contracts, managing financial transactions, or making decisions regarding the person’s property or affairs. The attorney in fact operates under the principle of fiduciary duty, meaning they are required to act in the best interests of the person they represent.

While "power of attorney" refers to the document that grants that authority, it does not specifically denote the person exercising that power. The terms "client" and "customer" generally refer to individuals receiving services or purchasing goods, rather than someone who acts on behalf of another party in signing documents. Thus, "Attorney in Fact" is the most accurate term in this context.