The Agnus Dei (Latin for "Lamb of God") may appear in several postures.
Seated with a banner, the Agnus Dei represents the risen Christ who triumphs over death. This symbol is rich in significance. John the Baptist proclaimed Jesus to be the Lamb of God who takes away the sin of the world. In the Revelation, Jesus is portrayed as a lamb. Even in the Old Testament, God's provision of a ram as a substitute sacrifice for Isaac is an important type of Christ.
John 1:29
The next day John saw Jesus coming toward him and said, "Look, the Lamb of God, who takes away the sin of the world!
Seated on a book with seven seals, it represents the final judgment when Christ returns in glory.
Rev. 5:11
Then I looked and heard the voice of many angels, numbering thousands upon thousands, and ten thousand times ten thousand. They encircled the throne and the living creatures and the elders. 12 In a loud voice they sang: "Worthy is the Lamb, who was slain, to receive power and wealth and wisdom and strength and honor and glory and praise!"