Secondary residence here doesn't mean your 2nd house as opposed to the 3rd, it means a vacation home (ie a place you own for your own use, even if it's not you main residence. As opposed to a rental property)

It's not just that.

Morada is any property that you live, use to live or use to do private activities in it.

If you used at least once a year or so, then is morada. Like if it was your primary house.

If you have a semi demolished house, or unused house during years, then clearly is it not a morada. That's the difference.