Why is V_ECO very low (0.3V) for Darlington transistors such as TLP187 and TLP387? What precautions should be taken when using such transistors?

When a transistor in a Darlington configuration with base-emitter resistance is incorporated into an Si chip, stray diode Di is put between the emitter and the collector in an equivalent manner. Owing to this, for V_ECO, the characteristics in the forward direction of the Di also appear; V_ECO  is thus specified with a very low (0.3V) value. Through this Di element, a current flows between the emitter and the collector. The circuit must be designed so that the V_ECO value (0.3V) will not be exceeded.