When specified on our site, the surface field or magnetic field refers to the strength in Gauss. For axially magnetised discs and cylinders, it is specified on the surface of the magnet, along the centre axis of magnetization. For blocks, it is specified on the surface of the magnet, also along the centre axis of magnetization. For rings, you may see two values. B_y,center specifies the vertical component of the magnetic field in the air at the centre of the ring. B_y,ring specifies the vertical component of the magnetic field on the surface of the magnet, mid-way between the inner and outer diameters. Some depictions of magnet fields can be found here.