CDD - this is a city/regional issue and at least in Colorado, it can be different between different MLS's - most MLS's have a definition for this very issue. In Denver, the "Square Feet field" includes just main floor and second (and third) floor square footage. A walkout basement square footage would be included in the "Total Square Feet" field, in the "Basement Square Feet" field, and in the "Total Finished Square Feet" field (if the basement was finished). So you would need to ask an agent familiar with your area - some agents will include the walkout basement square feet, particularly if they are not aware of what the definitions are, or if they are trying to make their Seller's home look bigger than it really is - at least in the Denver area, an agent can be fined for misrepresenting the home if they do this.
In my experience, if the square footage listed is calculated differently than all the other homes that the Buyer is looking at, all it does is annoy the Buyer because the home it is much smaller than the other homes they are considering. I hope that helps.