Imagine this scenario: you are approaching the age at which you will begin to draw Old Age Security (OAS) and you face a choice whether to crystallize capital gains by selling securities in your taxable accounts now or after you begin to draw OAS.
In a nutshell: OAS is clawed back once net income passes $77,580 (in 2019) at a rate of 15 cents per additional dollar earned. Capital gains increase your net income – even if you have capital losses carried forward from previous years. This is because OAS clawback is calculated based on your net income before adjustments (line 234 of your tax return), and capital losses carried forward are deducted later (on line 253).
One way this could really matter: if you have carried forward losses, you may want to consider taking capital gains before you begin to draw OAS, and use the losses to offset the gains. Even if you do not have carried forward losses, you may benefit by taking capital gains and paying the resulting tax before you draw OAS; taking those capital gains while on OAS will trigger tax and potentially result in clawback of your OAS (an implicit tax).
Your tax situation is unique to you. A host of other factors may play into trading decisions. Be sure to discuss your tax strategy with your investment advisor and your accountant.