The superficial loss rule is a Canadian tax rule (CRA section 40(2)(g)(i)) designed to prevent fake tax-loss harvesting. If you sell a security at a loss and buy it back within 30 days (either directly, via your spouse, or in your RRSP/TFSA), the CRA disallows the loss for tax purposes.
Instead, the disallowed loss is added to the ACB of the replacement security. You don't lose the benefit forever, but you can't claim it against capital gains in the current year. This catches a lot of investors by surprise, especially those who sell in taxable and rebuy in TFSA. That still triggers the rule.
The 30-day window is on both sides of the sale. So if you sell AAPL at a loss on November 15, the rule covers any AAPL purchase from October 16 through December 15. Buying a substantially-identical security (e.g., a different AAPL class) may also trigger it.
WealthWatch scans your entire transaction history and flags every potential superficial loss automatically, across all your accounts and across spouse accounts if you use Family Sharing. You'll know before you file.