Ray threshold is set as a percentage value. The ray threshold algorithm is constantly evaluating a ray's contribution to the final pixel being rendered. Once the contribution of that ray gets below the ray threshold percentage it is eligible for termination. This does not mean that the ray will definitely be terminated. Rather it only indicates to the render engine that it is a candidate for termination. Wholesale terminating rays below this importance factor creates a biased effect that affects the overall lighting in a detrimental way. However, with each bounce or refraction that the ray undergoes after going under the ray threshold, the more likely that this ray will be terminated.