Second Question: What is the meaning of the affirmation that the Church of Christ subsists in the Catholic Church?Response: Christ "established here on earth" only one Church and instituted it as a "visible and spiritual community", that from its beginning and throughout the centuries has always existed and will always exist, and in which alone are found all the elements that Christ himself instituted. "This one Church of Christ, which we confess in the Creed as one, holy, catholic and apostolic […]. This Church, constituted and organised in this world as a society, subsists in the Catholic Church, governed by the successor of Peter and the Bishops in communion with him".
In number 8 of the Dogmatic Constitution Lumen gentium ‘subsistence’ means this perduring, historical continuity and the permanence of all the elements instituted by Christ in the Catholic Church, in which the Church of Christ is concretely found on this earth. It is possible, according to Catholic doctrine, to affirm correctly that the Church of Christ is present and operative in the churches and ecclesial Communities not yet fully in communion with the Catholic Church, on account of the elements of sanctification and truth that are present in them. Nevertheless, the word "subsists" can only be attributed to the Catholic Church alone precisely because it refers to the mark of unity that we profess in the symbols of the faith (I believe... in the "one" Church); and this "one" Church subsists in the Catholic Church.
Closing blockquote tag in case it's open