2 separate types of tax events here- the RTSO is based on income tax rates not capital gains tax, so whenever you acquired the shares, you owe income tax, usc and prsi on the gain at that moment. This becomes the taxable value for cgt purposes whenever you later sell. If you are still holding on to these shares, and they have dropped in value, there is no taxable cgt event yet. If you sold at a much lower value, then you won’t owe any cgt on the sale, and will have a cgt loss you can carry forward against subsequent cgt gains.