I said it before, looking at documentation will answer many of your questions.
https://www.w3.org/TR/CSS21/selector.html#class-html
To match a subset of "class" values, each value must be preceded by a ".".
For example, the following rule matches any P element
whose "class" attribute has been assigned a list of
space-separated values that includes "pastoral" and "marine":
p.marine.pastoral { color: green }
This rule matches when class="pastoral blue aqua marine"
but does not match for class="pastoral blue".