Hello, nhungthientai.
Overlay means that nothing will be deleted in the .bmco document during a manually-initiated import. A
normal import can be made to do this too if you dig into
Settings >
Clients, click the
advanced (gear icon) button and in the sheet which appears, switch off
Delete unmatched items.
It does not give you a
blob of data at the root level, as, for example, Safari does when you are running Safari and click in the menu:
File >
Import bookmarks. Instead, it maps items into the corresponding locations: Bookmarks Bar, Bookmarks Menu, etc.
You can filter only items from a particular browser (client) by clicking the tab:
Settings >
Reports >
Find/Replace, and configuring the filter for
All items whose
Client Associations contains the name of the particular browser.
You should be aware of the meanings of
import and
export:
Note: The above image does not work in Firefox 66 Beta for some reason. But it works in Safari. It's also in the Help Book.
As you can see,
Import can only affect the .bmco document. It is easy to undo, though, if something happens which you do not like, click in the menu:
Edit >
Undo Import. You can also restore prior versions using the macOS Versions Browser. To use that, click in the menu:
File >
Revert to
.
Export can only affect the web browser(s). This is more difficult to undo. So be sure before you export.