I presume that the question is related to the theory of quotation and shall set down several remarks that might be of help approaching the issue.
For technical matters involving formal languages, usually single quotation marks are used, and double quotation marks are mostly reserved to the verbal quotations, as for example,
- Marx says "Alles gesellschaftliche Leben ist wesentlich praktisch."
So, I'll stick to single quotation marks. As for the cases concerning the punctuation of a natural language, no doubt, the conventions of that language should be employed.
As a general principle, when an expression is mentioned as opposed to used, it is enclosed between quotation marks. Hence, as a philosophical question, quotation is most related to use-mention distinction. However, discrimination of use and mention cases is a matter of not absolute, but a relative matter. To put crudely, within a particular discourse, we identify an object level and a meta-level, then decide on when an expression is used (at object level) or mentioned (at meta-level).
But, on the one hand, the cases in actual practice of language are not so clear-cut, using and mentioning are complex linguistic devices, furthermore, quotation and mention are not equivalent (a sentence may well be mentioned without quoted, etc.). On the other hand, a hierarchy of use-mention instances can readily occur and get tangled:
In sentence 1, the expression E is used;
Sentence 2 mentions the expression E used in sentence 1;
Sentence 3 uses the mention in sentence 2 of the expression E;
. . .
Last but not least, an expression can be mentioned without quoted (viz. enclosed between quotation marks), hence, there is not a one-to-one between mention and quotation.
For now, I suppose, it is sufficient to be aware of such problematic facets of quotation and leave its ramifications to separate questions. For the present question, let us go over some examples and draw distinctions from them (the statements succeeding the examples are those that can be said on the basis of the example):
- 'The president of USA' := John
John is a sentence with 17 letters and 3 spaces.
- 'The president of USA' := 'John'
In each case the string of symbols 'The president of USA' occurs, it is replaceable with the string of symbols 'John'.
- The president of USA := John
The sentences 'The president of USA plays football' and 'John plays football' are referentially equivalent.
- (The name) 'John' designates the president of US.
Neither the person, nor the string of symbols, but the name 'John' (a noun of the language) refers to the president of USA (notice the conflicting situation: why not the string of symbols, but the name? That is why sometimes the phrase 'the name' is added to clarify the intended meaning).
- ⌜Prn is the president of USA⌝ is a sentence with a pronoun in the subject position.
Assuming Prn is a variable, other pronouns can legitimately replace it. The sentences that can immediately follow from the given one:
- 'He is the president of USA' is a sentence with a pronoun in the subject position.
- 'Someone is the president of USA' is a sentence with a pronoun in the subject position.
- 'She is the president of USA' is a sentence with a pronoun in the subject position.