Both source files and the current browser tree stay unchanged. You decide every identity match and contested link before downloading a new tree.
Compare their relatives and portraits, then choose. The confidence percentage helps order suggestions; it is not proof of identity.
The same link in both trees is combined. A link present in only one tree is kept by default and can be removed below. Only conflicting parents, partners or details of the same link require a decision.
These links are kept automatically. Remove one only if it is incorrect; this does not remove either person.