The LOCAL_PREF attribute is a well-known attribute that should be included in every route advertised to an IBGP or confederation-EBGP peer. It is used to influence the flow of outbound traffic from the AS. The local preference is a 32-bit value and higher values are more preferred by the BGP decision process. The LOCAL_PREF attribute is not included in routes advertised to EBGP peers (if the attribute is received from an EBGP peer it is ignored).
In SR OS the default local preference is 100 but this can be changed with the local-preference command or using route policies. When a LOCAL_PREF attribute needs to be added to a route because it does not have one (for example, because it was received from an EBGP peer) the value is the configured or default local-preference unless overridden by policy.