This question already has an answer here:
My book says that $\subset$ is used to represente any subset, proper or improper, needing in this case to show the anti symmetric property of sets. ($A = B \iff A \subset B \, \, \wedge \,\, B \subset A)$
And $\subseteq$ is used specifically to represent improper subsets. (In other words, $A \subseteq B \iff A = B) $
But i saw so many articles using $\subseteq$ and not $\subset$ then i am kinda suspicious about this definition.
If this definition is correct, why $\subseteq$ it's so used? Just because it helps to prove the equality betwen the sets?
Thanks for the help.