BSF is hosted in a network element under the control of an Mobile Network Operator (MNO).
BSF, HSS and UE participate in Generic Bootstraping Architecture (GBA) in which a shared secret is established between the network and a UE by running the bootstrapping procedure. The shared secret can be used between Network Application Functions (NAF's) and UE's,
E.g. : For authentication purposes
For Specification TS 33.220