A client-professional agreement (agency-style relationship for certain tasks) does not grant unlimited authority. NPPE contract and agency principles stress that a professional must act within the authority granted by the agreement and instructions, and must not exceed that authority or bind the client beyond what is authorized. This includes staying within the defined scope of services, decision rights, spending authority, and signing authority. Option C correctly captures this fundamental requirement. Option A is incorrect because professionals do not act for clients "in all aspects"; authority is limited and defined. Option B is incorrect because signing documents depends on delegated authority; professionals should not sign documents unless authorized and competent to do so. Option D may be a good practice in some engagements, but it is not a universal legal requirement of client-professional agreements. Therefore, C is correct.