There are 4 boolean flags which influence the import behaviour:
The latter two have been added with
SLING-8243. Currently the behaviour differs between JSON and XML Import and also from the Javadoc
the differences between overwrite and merge are not really clear (https://github.com/apache/sling-org-apache-sling-jcr-contentloader/blob/master/src/main/java/org/apache/sling/jcr/contentloader/ImportOptions.java).
For me the flag merge seems a bit misleading, as the javadoc indicates this will replace all old content (nodes and properties or only nodes). Usually when you refer to a merge you also keep some part of the existing content. It seems that merge should rather be named something like replace. WDYT?