What is the difference between eligible and non-eligible RDTOH?

RDTOH is actually split into two separate pools: eligible RDTOH and non-eligible RDTOH, and which one gets refunded depends on the type of dividend your corporation pays. Eligible RDTOH generally relates to refundable tax connected to eligible dividends the corporation itself received from other corporations and certain income taxed at the general corporate rate, while non-eligible RDTOH generally relates to refundable tax on investment income taxed at the corporation's regular, higher rate, along with non-eligible dividends the corporation received.

The practical difference shows up when your corporation decides what kind of dividend to pay: paying an eligible dividend to shareholders generally draws down the eligible RDTOH pool, while an ordinary, non-eligible dividend generally draws down the non-eligible pool first, following specific ordering rules built into the system. Since eligible and non-eligible dividends are also taxed differently in the shareholder's hands, the choice of which pool to draw down isn't purely mechanical, it interacts with the shareholder's own personal tax position too.

Because the two pools don't automatically substitute for each other, and mismanaging which dividend type is paid can leave one pool refunded while the other sits untouched, tracking both separately, rather than treating RDTOH as one single number, is important for getting full value out of the account.