Hello Carl,
Our DATA team provided the details around how to calculate these two metrics using Bulk Data API: Unique Visitors: In LSI, we are using HyperLogLog++ (HLL++) approximation algorithm, to calculate UniqueVisitors. To replicate it using Bulk Data API, you've to filter out "action.key=view" and apply HLL on "visitor.id"; This implementation is available in ElasticSearch. Visitors: In LSI, we are counting visits by using "action.key=view" and unique count of "visit.id". This implementation is available in ElasticSearch. Please, let us know if you need any further clarification.
Kind regards, Damian