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.

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

Difference Between Partly Cloudy and Mostly Sunny

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

Sunday's weather: Mostly cloudy with some sunny breaks, high of 35

Ask the meteorologist: What's the difference between mostly cloudy and partly sunny or partly cloudy and mostly sunny?

Ask Ellen: What's the difference between partly cloudy, partly sunny?

Seattle FAQ News, Weather, Sports, Breaking News

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

Partly Cloudy vs Partly Sunny - Difference and Comparison

What's the Difference Between “Mostly Sunny” and “Partly Cloudy”?

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