The skimpy docs on select-rule has come up before: http://forum.mikrotik.com/t/documentation-on-ros-v7-routing-select-rules/174640/1
It can’t hurt to open a ticket at support@mikrotik.com about the poor docs on BGP’s select-rule. I’m not sure what is not covered by the V7 BGP filter language e.g. what can only be done in select-rule, since the BGP rules support jump and if.