CFlist stands for Channel Frequency list. CFList is a list of five channel frequencies for the channels three to seven whereby each frequency is encoded as a 24 bits unsigned integer (three octets).
LoRaWAN implements this CFList of 16 bytes in the JoinAccept message.
The CFList is optional and its presence can be detected by the length of the join-accept message.
If present, the CFList SHALL replace all the previous channels stored in the end-device apart from the three default channels.
The newly defined channels are immediately enabled and usable by the end-device for communication.
The format of the CFList is region dependent.
Refer to the LoRaWan Regional Parameters v1.0 specification to get details about these 16 bytes.

Since Server software release 3.2.0, the CFlist is automatically generated by the server and added in the joinAns message. This CFlist is generated from the radio configuration of the gateway from which the joinRequest has been transmitted (in fact, the gateway that has the best SNR). If you fill the CFList manually, this CFlist will replace the one generated by the server.