It seems to me 523910 suits day trading when day traders act as principals that buy and sell for their own account. See the definition in NAICS Canada 2022 v1.0.
523910 - Miscellaneous intermediation
This Canadian industry comprises establishments primarily engaged in acting as principals (except investment bankers, securities dealers, and commodity contracts dealers) in buying or selling of financial contracts, generally on a spread basis. Principals are investors that buy or sell for their own account. This industry also covers legal entities, often referred to as personal holding companies, set up primarily for the purpose of holding, in a generally passive manner, the wealth of an individual or a family.
