Symbols of the Holy Spirit
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 Holy Orders, the image of the seal (sphragis) has been used in some theological traditions to express the indelible "character" imprinted by these three unrepeatable sacraments.
Source: Catechism of the Catholic Church (Vatican.va)