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As per CRA, If you have a capital loss in 2020, you can use it to reduce any capital gains you had in the year, to a balance of zero. If your capital losses are more than your capital gains, you may have a net capital loss for the year. You can use a net capital loss to reduce your taxable capital gains in any of the 3 preceding years or in any future year.
Therefore, the balance for capital gain field 0 is right. You cannot create a negative balance. You can use a net capital loss to reduce your taxable income in any future year.
Please visit the website on What happens if you have a capital loss? to learn more about capital loss.
But where does it show when we review the whole thing ? It's not clear and I would like to make sure that my 2020 capital loss is properly registered and will be added up to my cumulative Net Capital Losses from previous years...