This allows the concatenation or joining together of files vertically, allow to append horizontally, as well as filter via select/omit based on keys.
A Union is the base set. Anything in that union will be the left most table. If another file is joined with the base union set, all records of the base union will be retained and the joined file will be a left join which means retaining all records on the left hand side and any appends can be added such as DOB, DOD, or whatever you want to append.
Each merge will need an output schema and if also doing an omit/select, a key is needed and the box checked in the output field.
Go to Bulk Actions and Add to a Merge job for additional information.