This is such a great question! The way that MLS calculates "official" square footage is by adding together all of the floor areas of the levels of the home that are "above-grade", excluding the stairs.
This means that even if your basement is developed, the floor space is "below-grade".
Make sure to note in your description when you have a developed basement that there is additional "live-able square-footage" below grade.
At Bōde, we currently cover the cost of professional measurements that are needed to list your home on realtor.ca - this report dictates the square feet that will go on your listing. Alas, there is no wiggle room here - it is required by the industry. Rest assured all home measurements are taken the same way.