Catechism of the Catholic Church §698 — excerpt
The seal is a symbol close to that of anointing. "The Father has set his seal" on Christ and also seals us in him. Because this seal indicates the indelible effect of the anointing with the Holy Spirit in the sacraments of Baptism, Confirmation, and…
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.