Grandfathered cost
Regulation & taxThe cost substituted for what you actually paid, on listed equity and equity fund units acquired before 1 February 2018, so that gains accrued while such gains were exempt are not brought into charge.
In plain terms
It is the higher of your actual cost and the lower of the 31 January 2018 value and your sale price. The inner cap means it can never manufacture a loss; the outer floor means it can never remove a real one. Sell between what you paid and the 2018 value and the gain is exactly nil.