CAS is IAS corrected for instrument & position error. CAS is shown in the flight deck
To be precise, IAS is the value read on the dial of the airspeed indicator, corrected for instrument error by reference to the calibration of the instrument against a known pressure source.
CAS is IAS corrected for errors in the pitot and static pressures. The static pressure error is called 'position error' because it attributable to the position of the static pressure source. The pitot pressure error is mainly due to the water drains in the pitot tube, but can become greater at extreme angles of attack.
Large modern airplanes use an air data computer (ADC) or Air Data/Inertial Reference Unit (ADIRU) to correct the measured pitot/static pressures for the known errors, and to calculate the CAS that is shown on the flight deck. On those airplanes IAS equals CAS, provided the airplane stays within the envelope for which the pitot and static pressure errors are known.