Business Activity Tax Simplification Act of 201 8 This bill expands the federal prohibition against state taxation of interstate commerce to include taxation of out-of-state transactions involving all forms of property and services, including the furnishing or gathering of information and sales or transactions involving digital goods or services. (Under current law, the prohibition applies only to sales of tangible personal property.) The bill also (1) prohibits state taxation of an out-of-state entity unless the entity has a physical presence in the taxing state, (2) sets forth criteria for determining physical presence in a state, and (3) specifies requirements for computing the tax liability of affiliated businesses operating in a state.