Replicas do not share analytics data with their primary index.
As events are not forwarded from a primary index to its replicas, analytics data remains specific to the index where events are triggered. Each event is solely tied to the index declared in the index parameter, therefore if an event is being triggered on a replica index you should pass this index in the parameter.
When viewing the Analytics for your indices, you can choose to view the Analytics for a specific index, or an aggregation of the primary and its replicas, as shown below:
If you plan on implementing Dynamic Re-Ranking on your primary index, but have a replica index that receives the majority of events, you can set the replica as the source index.
This means that events from the replica will be used to determine the behaviour of Dynamic Re-Ranking on the primary index.