It is dimensionless. A sonic log gives a dynamic value from the compressional and shear velocities. A core test that loads the plug gives a static value. The two are not interchangeable, and a stability or fracture model usually wants the static number, sometimes converted from the log with a local correlation.
Sandstones often fall near 0.2. Shales are typically higher. The ratio is one input to the horizontal stress calculated from the overburden, along with Young's modulus and the pore pressure. A ratio copied from a textbook rock, rather than measured on the formation in question, is a guess with a decimal point.