The Bearer setting allows you to restrict the current APN configuration to only be available when specific communication protocols are in use.
This setting may not be available or fully functional on all devices.
If not specified, this APN may be used regardless of the communication protocol currently in use. The actual communication protocol in use will depend on the capabilities of your device and the cell tower it is currently communicating with.
This setting does not allow you to control which communication protocols may be used (for this, see Network mode under Mobile network settings), only whether this APN configuration may be activated on a given protocol. It allows you, for example, to use one APN configuration to be selected when LTE is in use, and another APN configuration to be selected if eHRPD or EvDO is in use.
There is normally no need to set this and for maximum compatibility this should normally be set to Unspecified unless your carrier recommends otherwise.