The Code Of Securities in Box 15 of the T5008 slip should be filled in by the issuer of the slip. Most often the T5008 slip is issued when you have a disposition of shares - Code SHS.
For a complete list of these codes, see this link from CRA: https://www.canada.ca/en/revenue-agency/services/tax/businesses/topics/completing-slips-summaries/fi...
