Managing assets — such as managing rental properties — is considered labor, and the money you receive related to that labor in particular is considered labor income, not capital income.

Where someone runs a business that they also own the capital for, the income they receive from it is called mixed income, since it contains a mix of income from the value of the labor you provide and the value of the capital you use. The capital component of mixed labor is the only thing that is considered capital income.

So, for instance, figure out how much you would have to pay a property management company to do the work you do managing your rentals. That is roughly how much your labor is worth. Income you make in excess of that is capital income, as it is coming from the return on your assets, not your property management labor.