Now — just a moment to describe some conventions. There are many of these in mathematics. These are not firm mathematical rules, but just traditions. It makes it much easier for people reading your work to understand what you are trying to say.
So when you are writing up homework, or just describing what you are doing, then if you stick with these conventions people reading your work (including the person marking your exams) will know — “Oh
is that set they are talking about” and “
is an element of that set.”. On the other hand, if you use any old letter or symbol it is correct, but confusing for the reader. Think of it as being a bit like spelling — if you don’t spell words correctly people can usually still understand what you mean, but it is much easier if you spell words the same way as everyone else.