Tax Law, section 209(1)
A foreign corporation that maintains an office in New York State is engaged in an activity that makes it subject to tax. An office is any area, enclosure or facility that is used in the regular course of the corporate business. A salesperson's home, a hotel room, or a trailer used on a construction job site may constitute an office.
N.Y. Comp. Codes R. & Regs. Tit. 20 §§ 1-2.7