Why are there two current transfer ratios: CTR and CTR (Saturation)?

The current transfer ratio (CTR) has LED current I_F and transistor collector-emitter voltage V_CE dependencies. For typical transistor-output photocouplers, CTR is specified under the conditions I_F=5mA and V_CE=5V. However, V_CE=5V is the condition for the non-saturated area. In applications using photocoulers, ON and OFF digital signals are transmitted very often. In such a case, in the ON state, photocouplers go into the saturated area. Therefore, the saturated condition (In Toshiba's case, V_CE=0.4V or 0.3V) may be also set in the CTR requirements, and the CTR under this condition is defined as CTR (Saturation).

Example: Case of transistor-output photocoupler TLP385