What is the current transfer ratio?

The current transfer ratio (CTR) refers to the ratio of the collector current at the output side I_c to the input current passed to the LED at the input side I_F expressed as a percentage. It is defined by the following formula.
CTR＝(I_C/I_F)×100 (%)
As is the case with hFE for transistors, the current transfer ratio is an important parameter for transistor-output photocouplers. The value of this current transfer ratio has temperature Ta, LED current I_F, and transistor collector-emitter voltage V_CE dependencies. In addition, when an LED is energized for a long time, its optical output degrades gradually. Therefore, in designing, such deterioration over time must be taken into account.
Refer to Toshiba Application Note "Basic Characteristics and Application Circuit Design of Transistor Couplers", Chapter  1. Basic Characteristics.
(https://toshiba.semicon-storage.com/ap-en/semiconductor/knowledge/application-note.html)