Bahais hold to progressive revelation: that God has disclosed himself through a succession of messengers, Abraham, Moses, Krishna, Zoroaster, the Buddha, Jesus, Muhammad, the Bab and Bahaullah among them, each suited to the capacity of an age, each renewing the same religion under different social law. On that view the Aqdas supersedes the ritual and legal provisions of earlier scriptures for this age without denying that those scriptures were revealed, and Bahaullah himself is not the last, a further messenger being expected after not less than a thousand years.
The Aqdas is not the whole of Bahai scripture, or close to it. Bahaullah's written output was enormous, running to thousands of tablets and letters; the doctrinal work Kitab-i-Iqan, the ethical Hidden Words and the mystical Seven Valleys are all held equally revealed. So are the writings of the Bab, executed at Tabriz in 1850, whose Persian Bayan the Aqdas both fulfils and abrogates, and those of Abdul-Baha, Bahaullah's son.
What keeps that body of text from splintering is the Covenant. Bahaullah's will, the Kitab-i-Ahd, appointed Abdul-Baha as his successor and as the authorised interpreter of his writings; Abdul-Baha's own will appointed Shoghi Effendi as Guardian with the same interpretive authority. Since 1963 authority has rested with the Universal House of Justice, which Bahais hold can legislate but not interpret revelation. The line matters because it is how the community avoided the schisms that followed comparable successions elsewhere: rival claimants appeared at each transition, including Bahaullah's half-brother Mirza Yahya and his own son Mirza Muhammad Ali, and each attracted only a small and short-lived following.