I often see manuscripts where the author is following the US style of double quotation marks around quotes from the sources they’re citing, but using single quotation marks as “scare quotes” or to refer to a word or phrase. Formal US English doesn’t make such a distinction—single quotation marks are only used for a quote within a quote.
(And for headlines in AP style, and in other cases I’m not concerned with here. Don’t @ me.)
If you’re used to relying on a double/single distinction to indicate which phrases in quotation marks are someone else’s words and which are your own, be aware that a copyeditor will probably turn them all into double quotation marks. This means you should write and format your text in such a way that the reader can make the distinction without that visual cue.
In the following example, the author quotes twice from the book being discussed (a mid-twentieth-century book on women’s colleges), then puts single scare quotes around superior to show critical distance from this idea of women having a special gift for understanding men’s intellectual or artistic work (ugh).
Deeming cultural creativity “vastly overrated,” he calls for women’s “sense of persons” and of relationships to be valued more highly, along with their ‘superior’ ability to understand the human significance of the art and science created by men.
A copyeditor would likely change those single quotes to double quotes, but this would make “superior” look like another quotation from the book being cited, which it isn’t. To avoid this ambiguity, the author could signal that critical distance without using scare quotes at all: their supposedly superior ability, what he considers a superior ability, etc.
(Okay, this is a line from my dissertation, and I went with purportedly superior.)
Of course, while the occasional scare quote can be effective, we shouldn’t be overusing them. If you’ve noticed (or an editor or colleague has pointed out) a tendency to use too many scare quotes in your writing, check out this post from Tweed Editing.
For referring to a word or phrase, the Chicago Manual (7.63) gives the option of either italics or quotation marks. (One of the example sentences: The term critical mass is more often used metaphorically than literally.) In writing that discusses terms from other languages, I like to apply a rule of using italics to refer to non-English terms and quotation marks for English terms. Another example from the same section of CMOS: The Spanish verbs ser and estar are both rendered by “to be.” (This is different from the question of whether and when to italicize non-English terms when you’re using them rather than mentioning them, which I might get to in a later post.)