The cumulative net investment loss (CNIL) (deducted from your cumulative capital gains) measures lifetime losses from investments to the extent that they exceed lifetime income.
When the costs of an investment exceed the income from that investment, you are entitled to a tax deduction. However, if you subsequently sell the investment, you should not also be entitled to a tax-free capital gain.
Your CNIL reduces the amount of your cumulative gains limit for the year and may affect the allowable amount of your capital gains deduction.