To Allow aged out derivative beneficiary of F3 & F4 Family based petition to retain their parents old priority date.

F3 (married sons and daughters of U.S. citizens) and F4 (brothers and
sisters of U.S. citizens). Once a visa becomes available, the child who turns
21 years old “age-out” and can no longer join the parents as derivative
beneficiaries.Under the CSPA, when a visa number becomes available, the
amount of time the petition was pending before the USCIS is deducted from the
child’s actual age. If the adjusted age of the child is under 21, the child
may join the parents as derivative beneficiary.If not, the petition shall
automatically be converted to the appropriate category and the alien shall
retain the original priority date issued upon receipt of the original
petition.This way the aged-out children won’t have to go to the back of
the line for a new family-based preference.