"Some reduce to a meaningless formula the necessity of belonging to the true Church in order to gain eternal salvation." (Pius XII, Humani Generis, n. 27)
“Outside the Catholic Church, there is no salvation.”
What does this mean?
To simplify, Catholics are full members of the Catholic Church. Protestants, heretics and schismatics may start off as full members of the Catholic Church, but they lose that membership overtime due to heresy or schism. However, they are still subject to the Catholic Church.
So Catholics are members of the Catholic Church, whereas Protestants and Orthodox are subjects of the Catholic Church.
Attaining salvation can be more difficult for Orthodox because of schism, and especially Protestants because of heresy. Yet there is the factor of invincible ignorance to take into consideration. While Catholics cannot automatically presume, invincible ignorance, it cannot be dismissed either. Ultimately God decides these things upon one’s particular judgment at the end of one’s life.