LTE-NB : Multi Carrier Operation
NB-IoT can operate in multi carrier mode. In this case, a specific carrier is configured for initial connection setup + data transfer and other carriers are configured only for data communication. The carrier for the initial connection setup is called Anchor carrier and the other carriers are called Non-Anchor carrier.
When NB-IoT is operating in 'inband' mode, only a specific PRB within the legacy LTE System Band can be configured as Anchor carrier. The PRB index within LTE System BW are as follows.
In terms of physical channel composition, there are some differences between Anchor and Non-Anchor carrier.
NOTE : I created Figue 1 and 2 using Matlab LTE Toolbox based on Matlab 2018b
< Figure 1 : Physical Channel Composition - Anchor Carrier >
< Figure 2 : Physical Channel Composition - NonAnchor Carrier >
 Narrowband Internet of Things (R&S Whitepaper)