What is Merge out?

Mergeout is a Tuple Mover process that consolidates ROS containers and purges deleted records. There is no function called Merge into in Vertica.

@HyeontaeJu is probably refering to the MERGE SQL statement (i.e. MERGE INTO...)

Doc Page:
https://www.vertica.com/docs/latest/HTML/Content/Authoring/SQLReferenceManual/Statements/MERGE.htm

Merge out and Merge into are totally different and has no relation at all.
Remember Vertica creates new container per table per load. Merge out is to consolidate multiple containers into a single container for better read performance of database.
Merge into is a SQL statement that performs update (if data exists on target table) and insert (if data not exists on target table) at the same time to consolidate data from SQL level. You can refer details for merge into statement from the URL Jim has posted earlier.