On our types it's the closest approximation we can give to CAS within the limitations of the air data system and knowledge of the static and/or total source errors. Therefore it is IAS on all instruments, but the IAS on the electronic gadgets is usually rather closer to CAS than the IAS on the standby. Although with some fancy standby systems, there's a slightly dumb ADC incorporated, so even the standby 'IAS' is a decent stab at CAS.
It's not really practical to design a system for perfect airspeed indication, nor is it necessary.