The TFSA is intended as a savings and investment wrapper. Where the Canada Revenue Agency concludes the account is carrying on a securities trading business, the income becomes taxable in the trust, and the holder can be made jointly liable.
Audit activity has focused on accounts that grew enormously from small contributions through frequent options and small-cap trading, and courts have upheld assessments where the pattern matched the business factors set out under canada-capital-gains-inclusion-rate.
Practical implications: contributions are capped, over-contributions attract monthly penalty tax, and losses inside the account are never deductible, so aggressive trading there is asymmetric even before the reassessment risk.
General information about Canada, not tax advice. Rules change and depend on your circumstances; take professional advice before trading actively in a registered account.
Related: cra-canada, canada-capital-gains-inclusion-rate, rrsp, superficial-loss-rule, ciro