Refers to the punctuation mark '?' itself, or metaphorically to something that raises a question or doubt.
文の終わりに疑問符を付けてください。
Please put a question mark at the end of the sentence.
彼の説明には疑問符がつく。
His explanation raises a question.
Don't forget to put a question mark at the end of an interrogative sentence.
Listening to his explanation, a question mark formed in my mind.
That plan is still full of question marks, so it seems hard to carry out.
In English, questions usually end with a question mark.