"In order to find what change occured at the number 6, what exact command should be used?" - you'll need to compare it with the previous configuration that was present (e.g. was committed during the previous commit). In your case, it will be commit #7, and the correct answer is "show system rollback compare 7 6".
This example should make it clear - let's see what was committed during the last commit (#0):
[edit]
user@mx# set interfaces lo0.0 description test1
[edit]
user@mx# commit and-quit
commit complete
Exiting configuration mode
user@mx> show system commit
0 2019-10-10 09:48:28 CEST by user via cli
1 2019-10-08 11:32:30 CEST by root via other
user@mx> show system rollback compare 1 0
[edit interfaces lo0 unit 0]
+ description test1;
Best regards,
Sergii
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