With the default Diffuse = 2 sampling, you’ll get four diffuse rays for each camera ray. Those four diffuse rays are the first bounce after the camera ray “hits” a shape.

After the first bounce, Arnold sets all the sampling settings back to 1, so for each of those four diffuse rays, you get one second-bounce diffuse ray. This prevents an exponential explosion of rays as secondary rays like diffuse rays bounce around a scene.

The same thing is true for the other secondary rays such as glossy, refraction, and shadow rays.

So, in summary:

• For a camera ray you can get multiple secondary rays (for example, multiple diffuse rays or multiple glossy rays, and for light sampling, multiple shadow rays)
• But for a secondary ray, you’ll get just one ray. For example: one diffuse ray, one glossy ray (if any), one shadow ray, and so on.