The amount on a T5008 can be either, but it's usually simple to decide.
If someone is a casual investor and buying or selling stocks infrequently during the year, or only a few trades each month, then it's capital gains.
If someone is trading very often, such as "day trading" or many times per week or month, then CRA can decide that activity or that intention means the person is actually making their income by trading, so the T5008 should be income not capital gains.
There are more complex situations, for example if that T5008 was due to investments in a company where you are a partner, or if someone wants to specifically declare that T5008s in that year were income and not capital gains, for that you'd have to research more on the CRA site or get a tax professional to help. For casual investors just buy and selling on stock exchanges and not often enough for it to be their main income during the year though it's capital gains.
More info at https://www.canada.ca/en/revenue-agency/services/forms-publications/publications/it479r/archived-tra...
