3GPP TS 36.331 - allowedMeasBandwidth:
The IE AllowedMeasBandwidth is used to indicate the maximum allowed measurement bandwidth on a carrier frequency as defined by the parameter Transmission Bandwidth Configuration “NRB” TS 36.104 [47]. The values mbw6, mbw15, mbw25, mbw50, mbw75, mbw100 indicate 6, 15, 25, 50, 75 and 100 resource blocks respectively.
If absent, the value corresponding to the downlink bandwidth indicated by the dl-Bandwidth included in MasterInformationBlock applies.
allowedMeasBandwidth tells to UE that for a configured carrierfreq use either 6, 15, 25, 50, 75 or 100 RBs while reading RS from PDSCH channel.
UE can use this IE to get the dl-Bandwidth without MIB.