There are several ways cloud coverage is broken down. They can be broken into 10ths. Surface station plots break it down into 4ths.
One method of breaking down sky coverage is by: Sunny, Mostly Sunny, Partly Cloudy, Partly Sunny, Mostly Cloudy, and Overcast. The approximate amount of sky coverage for each of these descriptors would be:
0/10 clear / sunny
1/10 fair (often saved for high wispy cirrus)
2/10 to 3/10 mostly sunny
4/10 to 6/10 partly cloudy
7/10 to 8/10 mostly cloudy
10/10 cloudy / overcast
"Mixture of sun and clouds" and "variable clouds" are often terms that are used. These are used when the cloud cover percentage will change during the day or when cloud cover will be different across the forecast region. The sky condition can vary dramatically across a viewing area and changing weather can turn a sunny sky into a mostly cloudy sky very quickly.
I suspect the public has a good understanding of the terms cloudy, clear, mostly cloudy, and mostly clear. Partly cloudy is often used when sky condition is between 4/10ths and 6/10ths. This produces the following scale... clear 0/10, 1/10 to 3/10 mostly clear, 4/10 to 6/10 partly cloudy, 7/10 to 9/10 mostly cloudy, and 10/10 cloudy. This scale can be used both during the day and night. Terms such as sunny, partly sunny and mostly sunny work fine during the day but obviously can not be used at night.