What's the difference between 'Partly Sunny' and 'Mostly Cloudy'?

Partly sunny? Or Mostly cloudy? Sounds like the can be interchangeable but the National Weather Service has specific definitions on each term used to describe cloud cover.

Mostly cloudy skies tonight, windy conditions with a few remaining showers

Difference Between Partly Cloudy and Mostly Sunny

Intro Guide Part One: Mostly Sunny vs Partly Cloudy – janesweather

Why do people say partly cloudy and partly sunny like they're not the same thing? - Quora

Why do people say partly cloudy and partly sunny like they're not the same thing? - Quora

Cliff Mass Weather Blog: What is the difference between partly sunny and partly cloudy?

Matt's Weather Spot: Partly Cloudy vs. Partly Sunny

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Is it partly cloudy or mostly cloudy?