The separation property refers to the conclusion that

the determination of the best CAL is objective and the choice of the inputs to be used to determine the efficient frontier is subjective.

investors are separate beings and will therefore have different preferences regarding the risk-return tradeoff.

the choice of inputs to be used to determine the efficient frontier is objective and the choice of the best CAL is subjective.

the determination of the best risky portfolio is objective and the choice of the best complete portfolio is subjective.

the choice of the best complete portfolio is objective and the determination of the best risky portfolio is objective