Both cell barred and AC barred have different meaning.
Cell barred in SIB1 means cell is barred for all UEs or not. If SIB1 is broadcasting cell as barred then no UE can select/re-select the cell for attach.
AC barred in SIB2 is used to barred MO-data/MO-signalling for some of the UEs. whenever an UE is trying to any signalling then it first generates a random number between 0 and 1 then compares that value with the value of ac-BarringFactor value broadcast in SIB2. If the random value is less than the ac-BarringFactor then UE is allow to continue the signalling otherwise cell is barred for the ac-BarringTime.