How to use CSV Config Set's sharing mode to run thread in Jmeter?

Is there any way to use this option "edit" of "share mode" in CSV Data Set Config? I have found the description in official web site?

apache jmeter component CSV Data Set Config

Identifier - all threads sharing the same identifier share the same file. So for example if you have 4 thread groups, you could use a common id for two or more of the groups to share the file between them. Or you could use the thread number to share the file between the same thread numbers in different thread groups.

But I don't understand what is the mean of common id and thread number? How can I use them for the particular thread?