Styles defined at a high level (for instance a text style defined in a class attached to a container) works through on all elements within the container).
So if you have multiple paragraphs with text in your container they will all inherit the text style defined at the container level. No need to define the same text style for each separate paragraph.
At a lower level you may override a style defined at a higher level, so if you want a different style for text in only one of the paragraphs you select that paragraph and choose another text style.
In the text editor you may select single or multiple characters and override text style for your selection.
Of course, SD4 does not apply styles which it thinks are not appropriate for the selected element.
This hierarchy holds true for most html elements. It's not specific for Coffeecup software, it's part of the html / css specifications.
You don't have to go back to old sites to correct anything, if they do what you want them to do.
It's easy to see, once you see it.