For the notation $\displaystyle x \in A$,

Say $\displaystyle x \in A \Rightarrow x \in A \cup B$

Does $\displaystyle x \in A$ denote all element $\displaystyle x$ of $\displaystyle A$?

Take a different example:

Say $\displaystyle x \in A \Rightarrow x \in A - B$

Does $\displaystyle x \in A$ denote only some element $\displaystyle x$ of $\displaystyle A$?

How do you decide whether it's some $\displaystyle x $ or all $\displaystyle x$?