The documentation about the indices configuration lack information about how to use the Rules settings (condition, mapping, default) :
https://www.elastic.co/guide/en/beats/filebeat/5.0/elasticsearch-output.html#_indices

Some information was added in this topics on Filebeat forum :
https://discuss.elastic.co/t/set-indices-based-on-a-field-value/69579

Adding this information in the documentation will probably help users who want to use this feature.